On the expressive power of permanents and perfect matchings of matrices of bounded pathwidth/cliquewidth

Abstract: Some 25 years ago Valiant introduced an algebraic model of computation in order to study the complexity of evaluating families of polynomials. The theory was introduced along with the complexity classes VP and VNP which are analogues of the classical classes P and NP. Families of polynomials that are difficult to evaluate (that is, VNP-complete) includes the permanent and hamiltonian polynomials. In a previous paper the authors together with P. Koiran studied the expressive power of permanent and hamiltonian polynomials of matrices of bounded treewidth, as well as the expressive power of perfect matchings of planar graphs. It was established that the permanent and hamiltonian polynomials of matrices of bounded treewidth are equivalent to arithmetic formulas. Also, the sum of weights of perfect matchings of planar graphs was shown to be equivalent to (weakly) skew circuits. In this paper we continue the research in the direction described above, and study the expressive power of permanents, hamiltonians and perfect matchings of matrices that have bounded pathwidth or bounded cliquewidth. In particular, we prove that permanents, hamiltonians and perfect matchings of matrices that have bounded pathwidth express exactly arithmetic formulas. This is an improvement of our previous result for matrices of bounded treewidth. Also, for matrices of bounded weighted cliquewidth we show membership in VP for these polynomials.


On the Effects of Idiotypic Interactions for Recommendation Communities in Artificial Immune Systems

Abstract: It has previously been shown that a recommender based on immune system idiotypic principles can out perform one based on correlation alone. This paper reports the results of work in progress, where we undertake some investigations into the nature of this beneficial effect. The initial findings are that the immune system recommender tends to produce different neighbourhoods, and that the superior performance of this recommender is due partly to the different neighbourhoods, and partly to the way that the idiotypic effect is used to weight each neighbours recommendations.


A Recommender System based on the Immune Network

Abstract: The immune system is a complex biological system with a highly distributed, adaptive and self-organising nature. This paper presents an artificial immune system (AIS) that exploits some of these characteristics and is applied to the task of film recommendation by collaborative filtering (CF). Natural evolution and in particular the immune system have not been designed for classical optimisation. However, for this problem, we are not interested in finding a single optimum. Rather we intend to identify a sub-set of good matches on which recommendations can be based. It is our hypothesis that an AIS built on two central aspects of the biological immune system will be an ideal candidate to achieve this: Antigen - antibody interaction for matching and antibody - antibody interaction for diversity. Computational results are presented in support of this conjecture and compared to those found by other CF techniques.


The Danger Theory and Its Application to Artificial Immune Systems

Abstract: Over the last decade, a new idea challenging the classical self-non-self viewpoint has become popular amongst immunologists. It is called the Danger Theory. In this conceptual paper, we look at this theory from the perspective of Artificial Immune System practitioners. An overview of the Danger Theory is presented with particular emphasis on analogies in the Artificial Immune Systems world. A number of potential application areas are then used to provide a framing for a critical assessment of the concept, and its relevance for Artificial Immune Systems.


Partnering Strategies for Fitness Evaluation in a Pyramidal Evolutionary Algorithm

Abstract: This paper combines the idea of a hierarchical distributed genetic algorithm with different inter-agent partnering strategies. Cascading clusters of sub-populations are built from bottom up, with higher-level sub-populations optimising larger parts of the problem. Hence higher-level sub-populations search a larger search space with a lower resolution whilst lower-level sub-populations search a smaller search space with a higher resolution. The effects of different partner selection schemes for (sub-)fitness evaluation purposes are examined for two multiple-choice optimisation problems. It is shown that random partnering strategies perform best by providing better sampling and more diversity.


Shallow, Low, and Light Trees, and Tight Lower Bounds for Euclidean Spanners

Abstract: We show that for every $n$-point metric space $M$ there exists a spanning tree $T$ with unweighted diameter $O(\log n)$ and weight $\omega(T) = O(\log n) \cdot \omega(MST(M))$. Moreover, there is a designated point $rt$ such that for every point $v$, $dist_T(rt,v) \le (1+\epsilon) \cdot dist_M(rt,v)$, for an arbitrarily small constant $\epsilon > 0$. We extend this result, and provide a tradeoff between unweighted diameter and weight, and prove that this tradeoff is \emph{tight up to constant factors} in the entire range of parameters. These results enable us to settle a long-standing open question in Computational Geometry. In STOC'95 Arya et al. devised a construction of Euclidean Spanners with unweighted diameter $O(\log n)$ and weight $O(\log n) \cdot \omega(MST(M))$. Ten years later in SODA'05 Agarwal et al. showed that this result is tight up to a factor of $O(\log \log n)$. We close this gap and show that the result of Arya et al. is tight up to constant factors.


Multiparty Communication Complexity of Disjointness

Abstract: We obtain a lower bound of n^Omega(1) on the k-party randomized communication complexity of the Disjointness function in the `Number on the Forehead' model of multiparty communication when k is a constant. For k=o(loglog n), the bounds remain super-polylogarithmic i.e. (log n)^omega(1). The previous best lower bound for three players until recently was Omega(log n). Our bound separates the communication complexity classes NP^{CC}_k and BPP^{CC}_k for k=o(loglog n). Furthermore, by the results of Beame, Pitassi and Segerlind \cite{BPS07}, our bound implies proof size lower bounds for tree-like, degree k-1 threshold systems and superpolynomial size lower bounds for Lovasz-Schrijver proofs. Sherstov \cite{She07b} recently developed a novel technique to obtain lower bounds on two-party communication using the approximate polynomial degree of boolean functions. We obtain our results by extending his technique to the multi-party setting using ideas from Chattopadhyay \cite{Cha07}. A similar bound for Disjointness has been recently and independently obtained by Lee and Shraibman.


A path following algorithm for the graph matching problem

Abstract: We propose a convex-concave programming approach for the labeled weighted graph matching problem. The convex-concave programming formulation is obtained by rewriting the weighted graph matching problem as a least-square problem on the set of permutation matrices and relaxing it to two different optimization problems: a quadratic convex and a quadratic concave optimization problem on the set of doubly stochastic matrices. The concave relaxation has the same global minimum as the initial graph matching problem, but the search for its global minimum is also a hard combinatorial problem. We therefore construct an approximation of the concave problem solution by following a solution path of a convex-concave problem obtained by linear interpolation of the convex and concave formulations, starting from the convex relaxation. This method allows to easily integrate the information on graph label similarities into the optimization problem, and therefore to perform labeled weighted graph matching. The algorithm is compared with some of the best performing graph matching methods on four datasets: simulated graphs, QAPLib, retina vessel images and handwritten chinese characters. In all cases, the results are competitive with the state-of-the-art.


Merkle's Key Agreement Protocol is Optimal: An $O(n^2)$ Attack on any Key Agreement from Random Oracles

Abstract: We prove that every key agreement protocol in the random oracle model in which the honest users make at most $n$ queries to the oracle can be broken by an adversary who makes $O(n^2)$ queries to the oracle. This improves on the previous $\widetilde{\Omega}(n^6)$ query attack given by Impagliazzo and Rudich (STOC '89) and resolves an open question posed by them. Our bound is optimal up to a constant factor since Merkle proposed a key agreement protocol in 1974 that can be easily implemented with $n$ queries to a random oracle and cannot be broken by any adversary who asks $o(n^2)$ queries.


Regulation and the Integrity of Spreadsheets in the Information Supply Chain

Abstract: Spreadsheets provide many of the key links between information systems, closing the gap between business needs and the capability of central systems. Recent regulations have brought these vulnerable parts of information supply chains into focus. The risk they present to the organisation depends on the role that they fulfil, with generic differences between their use as modeling tools and as operational applications. Four sections of the Sarbanes-Oxley Act (SOX) are particularly relevant to the use of spreadsheets. Compliance with each of these sections is dependent on maintaining the integrity of those spreadsheets acting as operational applications. This can be achieved manually but at high cost. There are a range of commercially available off-the-shelf solutions that can reduce this cost. These may be divided into those that assist in the debugging of logic and more recently the arrival of solutions that monitor the change and user activity taking place in business-critical spreadsheets. ClusterSeven provides one of these monitoring solutions, highlighting areas of operational risk whilst also establishing a database of information to deliver new business intelligence.


Lower Bounds on Signatures from Symmetric Primitives

Abstract: We show that every construction of one-time signature schemes from a random oracle achieves black-box security at most $2^{(1+o(1))q}$, where $q$ is the total number of oracle queries asked by the key generation, signing, and verification algorithms. That is, any such scheme can be broken with probability close to $1$ by a (computationally unbounded) adversary making $2^{(1+o(1))q}$ queries to the oracle. This is tight up to a constant factor in the number of queries, since a simple modification of Lamport's one-time signatures (Lamport '79) achieves $2^{(0.812-o(1))q}$ black-box security using $q$ queries to the oracle. Our result extends (with a loss of a constant factor in the number of queries) also to the random permutation and ideal-cipher oracles. Since the symmetric primitives (e.g. block ciphers, hash functions, and message authentication codes) can be constructed by a constant number of queries to the mentioned oracles, as corollary we get lower bounds on the efficiency of signature schemes from symmetric primitives when the construction is black-box. This can be taken as evidence of an inherent efficiency gap between signature schemes and symmetric primitives.


Ensuring Spreadsheet Integrity with Model Master

Abstract: We have developed the Model Master (MM) language for describing spreadsheets, and tools for converting MM programs to and from spreadsheets. The MM decompiler translates a spreadsheet into an MM program which gives a concise summary of its calculations, layout, and styling. This is valuable when trying to understand spreadsheets one has not seen before, and when checking for errors. The MM compiler goes the other way, translating an MM program into a spreadsheet. This makes possible a new style of development, in which spreadsheets are generated from textual specifications. This can reduce error rates compared to working directly with the raw spreadsheet, and gives important facilities for code reuse. MM programs also offer advantages over Excel files for the interchange of spreadsheets.


Picking up the Pieces: Self-Healing in Reconfigurable Networks

Abstract: We consider the problem of self-healing in networks that are reconfigurable in the sense that they can change their topology during an attack. Our goal is to maintain connectivity in these networks, even in the presence of repeated adversarial node deletion, by carefully adding edges after each attack. We present a new algorithm, DASH, that provably ensures that: 1) the network stays connected even if an adversary deletes up to all nodes in the network; and 2) no node ever increases its degree by more than 2 log n, where n is the number of nodes initially in the network. DASH is fully distributed; adds new edges only among neighbors of deleted nodes; and has average latency and bandwidth costs that are at most logarithmic in n. DASH has these properties irrespective of the topology of the initial network, and is thus orthogonal and complementary to traditional topology-based approaches to defending against attack. We also prove lower-bounds showing that DASH is asymptotically optimal in terms of minimizing maximum degree increase over multiple attacks. Finally, we present empirical results on power-law graphs that show that DASH performs well in practice, and that it significantly outperforms naive algorithms in reducing maximum degree increase. We also present empirical results on performance of our algorithms and a new heuristic with regard to stretch (increase in shortest path lengths).


3D-Ultrasound probe calibration for computer-guided diagnosis and therapy

Abstract: With the emergence of swept-volume ultrasound (US) probes, precise and almost real-time US volume imaging has become available. This offers many new opportunities for computer guided diagnosis and therapy, 3-D images containing significantly more information than 2-D slices. However, computer guidance often requires knowledge about the exact position of US voxels relative to a tracking reference, which can only be achieved through probe calibration. In this paper we present a 3-D US probe calibration system based on a membrane phantom. The calibration matrix is retrieved by detection of a membrane plane in a dozen of US acquisitions of the phantom. Plane detection is robustly performed with the 2-D Hough transformation. The feature extraction process is fully automated, calibration requires about 20 minutes and the calibration system can be used in a clinical context. The precision of the system was evaluated to a root mean square (RMS) distance error of 1.15mm and to an RMS angular error of 0.61 degrees. The point reconstruction accuracy was evaluated to 0.9mm and the angular reconstruction accuracy to 1.79 degrees.


5-cycles and the Petersen graph

Abstract: We show that if G is a connected bridgeless cubic graph whose every 2-factor is comprised of cycles of length five then G is the Petersen graph.


Modular Compilation of a Synchronous Language

Abstract: Synchronous languages rely on formal methods to ease the development of applications in an efficient and reusable way. Formal methods have been advocated as a means of increasing the reliability of systems, especially those which are safety or business critical. It is still difficult to develop automatic specification and verification tools due to limitations like state explosion, undecidability, etc... In this work, we design a new specification model based on a reactive synchronous approach. Then, we benefit from a formal framework well suited to perform compilation and formal validation of systems. In practice, we design and implement a special purpose language (LE) and its two semantics: the ehavioral semantics helps us to define a program by the set of its behaviors and avoid ambiguousness in programs' interpretation; the execution equational semantics allows the modular compilation of programs into software and hardware targets (c code, vhdl code, fpga synthesis, observers). Our approach is pertinent considering the two main requirements of critical realistic applications: the modular compilation allows us to deal with large systems, the model-based approach provides us with formal validation.


Characterization of the Vertices and Extreme Directions of the Negative Cycles Polyhedron and Hardness of Generating Vertices of 0/1-Polyhedra

Abstract: Given a graph $G=(V,E)$ and a weight function on the edges $w:E\mapsto\RR$, we consider the polyhedron $P(G,w)$ of negative-weight flows on $G$, and get a complete characterization of the vertices and extreme directions of $P(G,w)$. As a corollary, we show that, unless $P=NP$, there is no output polynomial-time algorithm to generate all the vertices of a 0/1-polyhedron. This strengthens the NP-hardness result of Khachiyan et al. (2006) for non 0/1-polyhedra, and comes in contrast with the polynomiality of vertex enumeration for 0/1-polytopes \cite{BL98} [Bussieck and L\"ubbecke (1998)].


Robustness Evaluation of Two CCG, a PCFG and a Link Grammar Parsers

Abstract: Robustness in a parser refers to an ability to deal with exceptional phenomena. A parser is robust if it deals with phenomena outside its normal range of inputs. This paper reports on a series of robustness evaluations of state-of-the-art parsers in which we concentrated on one aspect of robustness: its ability to parse sentences containing misspelled words. We propose two measures for robustness evaluation based on a comparison of a parser's output for grammatical input sentences and their noisy counterparts. In this paper, we use these measures to compare the overall robustness of the four evaluated parsers, and we present an analysis of the decline in parser performance with increasing error levels. Our results indicate that performance typically declines tens of percentage units when parsers are presented with texts containing misspellings. When it was tested on our purpose-built test set of 443 sentences, the best parser in the experiment (C&C parser) was able to return exactly the same parse tree for the grammatical and ungrammatical sentences for 60.8%, 34.0% and 14.9% of the sentences with one, two or three misspelled words respectively.


Analysis of Prime Reciprocal Sequences in Base 10

Abstract: Prime reciprocals have applications in coding and cryptography and for generation of random sequences. This paper investigates the structural redundancy of prime reciprocals in base 10 in a manner that parallels an earlier study for binary prime reciprocals. Several different kinds of structural relationships amongst the digits in reciprocal sequences are classified with respect to the digit in the least significant place of the prime. It is also shown that the frequency of digit 0 exceeds that of every other digit when the entire set of prime reciprocal sequences is considered.


Comparison of Spreadsheets with other Development Tools (limitations, solutions, workarounds and alternatives)

Abstract: The spreadsheet paradigm has some unique risks and challenges that are not present in more traditional development technologies. Many of the recent advances in other branches of software development have bypassed spreadsheets and spreadsheet developers. This paper compares spreadsheets and spreadsheet development to more traditional platforms such as databases and procedural languages. It also considers the fundamental danger introduced in the transition from paper spreadsheets to electronic. Suggestions are made to manage the risks and work around the limitations.


Between conjecture and memento: shaping a collective emotional perception of the future

Abstract: Large scale surveys of public mood are costly and often impractical to perform. However, the web is awash with material indicative of public mood such as blogs, emails, and web queries. Inexpensive content analysis on such extensive corpora can be used to assess public mood fluctuations. The work presented here is concerned with the analysis of the public mood towards the future. Using an extension of the Profile of Mood States questionnaire, we have extracted mood indicators from 10,741 emails submitted in 2006 to futureme.org, a web service that allows its users to send themselves emails to be delivered at a later date. Our results indicate long-term optimism toward the future, but medium-term apprehension and confusion.


Encoding changing country codes for the Semantic Web with ISO 3166 and SKOS

Abstract: This paper shows how authority files can be encoded for the Semantic Web with the Simple Knowledge Organisation System (SKOS). In particular the application of SKOS for encoding the structure, management, and utilization of country codes as defined in ISO 3166 is demonstrated. The proposed encoding gives a use case for SKOS that includes features that have only been discussed little so far, such as multiple notations, nested concept schemes, changes by versioning.


On the Continuity Set of an omega Rational Function

Abstract: In this paper, we study the continuity of rational functions realized by B\"uchi finite state transducers. It has been shown by Prieur that it can be decided whether such a function is continuous. We prove here that surprisingly, it cannot be decided whether such a function F has at least one point of continuity and that its continuity set C(F) cannot be computed. In the case of a synchronous rational function, we show that its continuity set is rational and that it can be computed. Furthermore we prove that any rational Pi^0_2-subset of X^omega for some alphabet X is the continuity set C(F) of an omega-rational synchronous function F defined on X^omega.


Increased security through open source

Abstract: In this paper we discuss the impact of open source on both the security and transparency of a software system. We focus on the more technical aspects of this issue, combining and extending arguments developed over the years. We stress that our discussion of the problem only applies to software for general purpose computing systems. For embedded systems, where the software usually cannot easily be patched or upgraded, different considerations may apply.


Crossing Borders: Security and Privacy Issues of the European e-Passport

Abstract: The first generation of European e-passports will be issued in 2006. We discuss how borders are crossed regarding the security and privacy erosion of the proposed schemes, and show which borders need to be crossed to improve the security and the privacy protection of the next generation of e-passports. In particular we discuss attacks on Basic Access Control due to the low entropy of the data from which the access keys are derived, we sketch the European proposals for Extended Access Control and the weaknesses in that scheme, and show how fundamentally different design decisions can make e-passports more secure.


Framework for 3D TransRectal Ultrasound

Abstract: Prostate biopsies are mainly performed under 2D TransRectal UltraSound (TRUS) control by sampling the prostate according to a predefined pattern. In case of first biopsies, this pattern follows a random systematic plan. Sometimes, repeat biopsies can be needed to target regions unsampled by previous biopsies or resample critical regions (for example in case of cancer expectant management or previous prostatic intraepithelial neoplasia findings). From a clinical point of view, it could be useful to control the 3D spatial distribution of theses biopsies inside the prostate. Modern 3D-TRUS probes allow acquiring high-quality volumes of the prostate in few seconds. We developed a framework to track the prostate in 3D TRUS images. It means that if one acquires a reference volume at the beginning of the session and another during each biopsy, it is possible to determine the relationship between the prostate in the reference and the others volumes by aligning images. We used this tool to evaluate the ability of a single operator (a young urologist assistant professor) to perform a pattern of 12 biopsies under 2D TRUS guidance.


A Bayesian Optimisation Algorithm for the Nurse Scheduling Problem

Abstract: A Bayesian optimization algorithm for the nurse scheduling problem is presented, which involves choosing a suitable scheduling rule from a set for each nurses assignment. Unlike our previous work that used Gas to implement implicit learning, the learning in the proposed algorithm is explicit, ie. Eventually, we will be able to identify and mix building blocks directly. The Bayesian optimization algorithm is applied to implement such explicit learning by building a Bayesian network of the joint distribution of solutions. The conditional probability of each variable in the network is computed according to an initial set of promising solutions. Subsequently, each new instance for each variable is generated, ie in our case, a new rule string has been obtained. Another set of rule strings will be generated in this way, some of which will replace previous strings based on fitness selection. If stopping conditions are not met, the conditional probabilities for all nodes in the Bayesian network are updated again using the current set of promising rule strings. Computational results from 52 real data instances demonstrate the success of this approach. It is also suggested that the learning mechanism in the proposed approach might be suitable for other scheduling problems.


Spanners of Additively Weighted Point Sets

Abstract: We study the problem of computing geometric spanners for (additively) weighted point sets. A weighted point set is a set of pairs $(p,r)$ where $p$ is a point in the plane and $r$ is a real number. The distance between two points $(p_i,r_i)$ and $(p_j,r_j)$ is defined as $|p_ip_j|-r_i-r_j$. We show that in the case where all $r_i$ are positive numbers and $|p_ip_j|\geq r_i+r_j$ for all $i,j$ (in which case the points can be seen as non-intersecting disks in the plane), a variant of the Yao graph is a $(1+\epsilon)$-spanner that has a linear number of edges. We also show that the Additively Weighted Delaunay graph (the face-dual of the Additively Weighted Voronoi diagram) has constant spanning ratio. The straight line embedding of the Additively Weighted Delaunay graph may not be a plane graph. We show how to compute a plane embedding that also has a constant spanning ratio.


A Class of Convex Polyhedra with Few Edge Unfoldings

Abstract: We construct a sequence of convex polyhedra on n vertices with the property that, as n -> infinity, the fraction of its edge unfoldings that avoid overlap approaches 0, and so the fraction that overlap approaches 1. Nevertheless, each does have (several) nonoverlapping edge unfoldings.


Bounded Mean-Delay Throughput and Non-Starvation Conditions in Aloha Network

Abstract: This paper considers the requirements to ensure bounded mean queuing delay and non-starvation in a slotted Aloha network operating the exponential backoff protocol. It is well-known that the maximum possible throughput of a slotted Aloha system with a large number of nodes is 1/e = 0.3679. Indeed, a saturation throughput of 1/e can be achieved with an exponential backoff factor of r = e/(e-1)=1.5820. The binary backoff factor of r = 2 is assumed in the majority of prior work, and in many practical multiple-access networks such as the Ethernet and WiFi. For slotted Aloha, the saturation throughput 0.3466 for r = 2 is reasonably close to the maximum of 1/e, and one could hardly raise objection to adopting r = 2 in the system. However, this paper shows that if mean queuing delay is to be bounded, then the sustainable throughput when r = 2 is only 0.2158, a drastic 41% drop from 1/e . Fortunately, the optimal setting of r = 1.3757 under the bounded mean-delay requirement allows us to achieve sustainable throughput of 0.3545, a penalty of only less than 4% relative to 1/e. A general conclusion is that the value of r may significantly affect the queuing delay performance. Besides analyzing mean queuing delay, this paper also delves into the phenomenon of starvation, wherein some nodes are deprived of service for an extended period of time while other nodes hog the system. Specifically, we propose a quantitative definition for starvation and show that the conditions to guarantee bounded mean delay and non-starved operation are one of the same, thus uniting these two notions. Finally, we show that when mean delay is large and starvation occurs, the performance results obtained from simulation experiments may not converge. A quantitative discussion of this issue is provided in this paper.


The optimal assignment kernel is not positive definite

Abstract: We prove that the optimal assignment kernel, proposed recently as an attempt to embed labeled graphs and more generally tuples of basic data to a Hilbert space, is in fact not always positive definite.


An equivalence preserving transformation from the Fibonacci to the Galois NLFSRs

Abstract: Conventional Non-Linear Feedback Shift Registers (NLFSRs) use the Fibonacci configuration in which the value of the first bit is updated according to some non-linear feedback function of previous values of other bits, and each remaining bit repeats the value of its previous bit. We show how to transform the feedback function of a Fibonacci NLFSR into several smaller feedback functions of individual bits. Such a transformation reduces the propagation time, thus increasing the speed of pseudo-random sequence generation. The practical significance of the presented technique is that is makes possible increasing the keystream generation speed of any Fibonacci NLFSR-based stream cipher with no penalty in area.


On Reliability of Dynamic Addressing Routing Protocols in Mobile Ad Hoc Networks

Abstract: In this paper, a reliability analysis is carried out to state a performance comparison between two recently proposed proactive routing algorithms. These protocols are able to scale in ad hoc and sensor networks by resorting to dynamic addressing, to face with the topology variability, which is typical of ad hoc, and sensor networks. Numerical simulations are also carried out to corroborate the results of the analysis.


Quantified Propositional Logspace Reasoning

Abstract: In this paper, we develop a quantified propositional proof systems that corresponds to logarithmic-space reasoning. We begin by defining a class SigmaCNF(2) of quantified formulas that can be evaluated in log space. Then our new proof system GL^* is defined as G_1^* with cuts restricted to SigmaCNF(2) formulas and no cut formula that is not quantifier free contains a free variable that does not appear in the final formula. To show that GL^* is strong enough to capture log space reasoning, we translate theorems of VL into a family of tautologies that have polynomial-size GL^* proofs. VL is a theory of bounded arithmetic that is known to correspond to logarithmic-space reasoning. To do the translation, we find an appropriate axiomatization of VL, and put VL proofs into a new normal form. To show that GL^* is not too strong, we prove the soundness of GL^* in such a way that it can be formalized in VL. This is done by giving a logarithmic-space algorithm that witnesses GL^* proofs.


Strategic Alert Throttling for Intrusion Detection Systems

Abstract: Network intrusion detection systems are themselves becoming targets of attackers. Alert flood attacks may be used to conceal malicious activity by hiding it among a deluge of false alerts sent by the attacker. Although these types of attacks are very hard to stop completely, our aim is to present techniques that improve alert throughput and capacity to such an extent that the resources required to successfully mount the attack become prohibitive. The key idea presented is to combine a token bucket filter with a realtime correlation algorithm. The proposed algorithm throttles alert output from the IDS when an attack is detected. The attack graph used in the correlation algorithm is used to make sure that alerts crucial to forming strategies are not discarded by throttling.


Solving Min-Max Problems with Applications to Games

Abstract: We refine existing general network optimization techniques, give new characterizations for the class of problems to which they can be applied, and show that they can also be used to solve various two-player games in almost linear time. Among these is a new variant of the network interdiction problem, where the interdictor wants to destroy high-capacity paths from the source to the destination using a vertex-wise limited budget of arc removals. We also show that replacing the limit average in mean payoff games by the maximum weight results in a class of games amenable to these techniques.


e-Science perspectives in Venezuela

Abstract: We describe the e-Science strategy in Venezuela, in particular initiatives by the Centro Nacional de Calculo Cientifico Universidad de Los Andes (CECALCULA), Merida, the Universidad de Los Andes (ULA), Merida, and the Instituto Venezolano de Investigaciones Cientificas (IVIC), Caracas. We present the plans for the Venezuelan Academic Grid and the current status of Grid ULA supported by Internet2. We show different web-based scientific applications that are being developed in quantum chemistry, atomic physics, structural damage analysis, biomedicine and bioclimate within the framework of the E-Infrastructure shared between Europe and Latin America (EELA)


Algorithms for Temperature-Aware Task Scheduling in Microprocessor Systems

Abstract: We study scheduling problems motivated by recently developed techniques for microprocessor thermal management at the operating systems level. The general scenario can be described as follows. The microprocessor's temperature is controlled by the hardware thermal management system that continuously monitors the chip temperature and automatically reduces the processor's speed as soon as the thermal threshold is exceeded. Some tasks are more CPU-intensive than other and thus generate more heat during execution. The cooling system operates non-stop, reducing (at an exponential rate) the deviation of the processor's temperature from the ambient temperature. As a result, the processor's temperature, and thus the performance as well, depends on the order of the task execution. Given a variety of possible underlying architectures, models for cooling and for hardware thermal management, as well as types of tasks, this scenario gives rise to a plethora of interesting and never studied scheduling problems. We focus on scheduling real-time jobs in a simplified model for cooling and thermal management. A collection of unit-length jobs is given, each job specified by its release time, deadline and heat contribution. If, at some time step, the temperature of the system is t and the processor executes a job with heat contribution h, then the temperature at the next step is (t+h)/2. The temperature cannot exceed the given thermal threshold T. The objective is to maximize the throughput, that is, the number of tasks that meet their deadlines. We prove that, in the offline case, computing the optimum schedule is NP-hard, even if all jobs are released at the same time. In the online case, we show a 2-competitive deterministic algorithm and a matching lower bound.


Protecting Spreadsheets Against Fraud

Abstract: Previous research on spreadsheet risks has predominantly focussed on errors inadvertently introduced by spreadsheet writers i.e. it focussed on the end-user aspects of spreadsheet development. When analyzing a faulty spreadsheet, one might not be able to determine whether a particular error (fault) has been made by mistake or with fraudulent intentions. However, the fences protecting against fraudulent errors have to be different from those shielding against inadvertent mistakes. Faults resulting from errors committed inadvertently can be prevented ab initio by tools that notify the spreadsheet writer about potential problems whereas faults that are introduced on purpose have to be discovered by auditors without the cooperation of their originators. Even worse, some spreadsheet writers will do their best to conceal fraudulent parts of their spreadsheets from auditors. In this paper we survey the available means for fraud protection by contrasting approaches suitable for spreadsheets with those known from fraud protection for conventional software.


Computational Models of Spreadsheet Development: Basis for Educational Approaches

Abstract: Among the multiple causes of high error rates in spreadsheets, lack of proper training and of deep understanding of the computational model upon which spreadsheet computations rest might not be the least issue. The paper addresses this problem by presenting a didactical model focussing on cell interaction, thus exceeding the atomicity of cell computations. The approach is motivated by an investigation how different spreadsheet systems handle certain computational issues implied from moving cells, copy-paste operations, or recursion.


Spreadsheet Debugging

Abstract: Spreadsheet programs, artifacts developed by non-programmers, are used for a variety of important tasks and decisions. Yet a significant proportion of them have severe quality problems. To address this issue, our previous work presented an interval-based testing methodology for spreadsheets. Interval-based testing rests on the observation that spreadsheets are mainly used for numerical computations. It also incorporates ideas from symbolic testing and interval analysis. This paper addresses the issue of efficiently debugging spreadsheets. Based on the interval-based testing methodology, this paper presents a technique for tracing faults in spreadsheet programs. The fault tracing technique proposed uses the dataflow information and cell marks to identify the most influential faulty cell(s) for a given formula cell containing a propagated fault.


Movie Recommendation Systems Using An Artificial Immune System

Abstract: We apply the Artificial Immune System (AIS) technology to the Collaborative Filtering (CF) technology when we build the movie recommendation system. Two different affinity measure algorithms of AIS, Kendall tau and Weighted Kappa, are used to calculate the correlation coefficients for this movie recommendation system. From the testing we think that Weighted Kappa is more suitable than Kendall tau for movie problems.


Exact Feasibility Tests for Real-Time Scheduling of Periodic Tasks upon Multiprocessor Platforms

Abstract: In this paper we study the global scheduling of periodic task systems upon multiprocessor platforms. We first show two very general properties which are well-known for uniprocessor platforms and which remain for multiprocessor platforms: (i) under few and not so restrictive assumptions, we show that feasible schedules of periodic task systems are periodic from some point with a period equal to the least common multiple of task periods and (ii) for the specific case of synchronous periodic task systems, we show that feasible schedules repeat from the origin. We then present our main result: we characterize, for task-level fixed-priority schedulers and for asynchronous constrained or arbitrary deadline periodic task models, upper bounds of the first time instant where the schedule repeats. We show that job-level fixed-priority schedulers are predictable upon unrelated multiprocessor platforms. For task-level fixed-priority schedulers, based on the upper bounds and the predictability property, we provide for asynchronous constrained or arbitrary deadline periodic task sets, exact feasibility tests. Finally, for the job-level fixed-priority EDF scheduler, for which such an upper bound remains unknown, we provide an exact feasibility test as well.


On Affinity Measures for Artificial Immune System Movie Recommenders

Abstract: We combine Artificial Immune Systems 'AIS', technology with Collaborative Filtering 'CF' and use it to build a movie recommendation system. We already know that Artificial Immune Systems work well as movie recommenders from previous work by Cayzer and Aickelin 3, 4, 5. Here our aim is to investigate the effect of different affinity measure algorithms for the AIS. Two different affinity measures, Kendalls Tau and Weighted Kappa, are used to calculate the correlation coefficients for the movie recommender. We compare the results with those published previously and show that Weighted Kappa is more suitable than others for movie problems. We also show that AIS are generally robust movie recommenders and that, as long as a suitable affinity measure is chosen, results are good.


Investigating Artificial Immune Systems For Job Shop Rescheduling In Changing Environments

Abstract: Artificial immune system can be used to generate schedules in changing environments and it has been proven to be more robust than schedules developed using a genetic algorithm. Good schedules can be produced especially when the number of the antigens is increased. However, an increase in the range of the antigens had somehow affected the fitness of the immune system. In this research, we are trying to improve the result of the system by rescheduling the same problem using the same method while at the same time maintaining the robustness of the schedules.


Artificial Immune Systems (AIS) - A New Paradigm for Heuristic Decision Making

Abstract: Over the last few years, more and more heuristic decision making techniques have been inspired by nature, e.g. evolutionary algorithms, ant colony optimisation and simulated annealing. More recently, a novel computational intelligence technique inspired by immunology has emerged, called Artificial Immune Systems (AIS). This immune system inspired technique has already been useful in solving some computational problems. In this keynote, we will very briefly describe the immune system metaphors that are relevant to AIS. We will then give some illustrative real-world problems suitable for AIS use and show a step-by-step algorithm walkthrough. A comparison of AIS to other well-known algorithms and areas for future work will round this keynote off. It should be noted that as AIS is still a young and evolving field, there is not yet a fixed algorithm template and hence actual implementations might differ somewhat from the examples given here.


TER: A Robot for Remote Ultrasonic Examination: Experimental Evaluations

Abstract: This chapter: o Motivates the clinical use of robotic tele-echography o Introduces the TER system o Describes technical and clinical evaluations performed with TER


A Locked Orthogonal Tree

Abstract: We give a counterexample to a conjecture of Poon [Poo06] that any orthogonal tree in two dimensions can always be flattened by a continuous motion that preserves edge lengths and avoids self-intersection. We show our example is locked by extending results on strongly locked self-touching linkages due to Connelly, Demaine and Rote [CDR02] to allow zero-length edges as defined in [ADG07], which may be of independent interest. Our results also yield a locked tree with only eleven edges, which is the smallest known example of a locked tree.


It's Not What You Have, But How You Use It: Compromises in Mobile Device Use

Abstract: As users begin to use many more devices for personal information management (PIM) than just the traditional desktop computer, it is essential for HCI researchers to understand how these devices are being used in the wild and their roles in users' information environments. We conducted a study of 220 knowledge workers about their devices, the activities they performed on each, and the groups of devices used together. Our findings indicate that several devices are often used in groups; integrated multi-function portable devices have begun to replace single-function devices for communication (e.g. email and IM). Users use certain features opportunistically because they happen to be carrying a multi-function device with them. The use of multiple devices and multi-function devices is fraught with compromises as users must choose and make trade-offs among various factors.


Abstractions for biomolecular computations

Abstract: Deoxyribonucleic acid is increasingly being understood to be an informational molecule, capable of information processing.It has found application in the determination of non-deterministic algorithms and in the design of molecular computing devices. This is a theoretical analysis of the mathematical properties and relations of the molecules which constituting DNA, which explains in part why DNA is a successful computing molecule.


Biopsies prostatiques sous guidage \'echographique 3D et temps r\'eel (4D) sur fant\^ome. Etude comparative versus guidage 2D

Abstract: This paper analyzes the impact of using 2D or 3D ultrasound on the efficiency of prostate biopsies. The evaluation is performed on home-made phantoms. The study shows that the accuracy is significantly improved.


The Complexity of Power-Index Comparison

Abstract: We study the complexity of the following problem: Given two weighted voting games G' and G'' that each contain a player p, in which of these games is p's power index value higher? We study this problem with respect to both the Shapley-Shubik power index [SS54] and the Banzhaf power index [Ban65,DS79]. Our main result is that for both of these power indices the problem is complete for probabilistic polynomial time (i.e., is PP-complete). We apply our results to partially resolve some recently proposed problems regarding the complexity of weighted voting games. We also study the complexity of the raw Shapley-Shubik power index. Deng and Papadimitriou [DP94] showed that the raw Shapley-Shubik power index is #P-metric-complete. We strengthen this by showing that the raw Shapley-Shubik power index is many-one complete for #P. And our strengthening cannot possibly be further improved to parsimonious completeness, since we observe that, in contrast with the raw Banzhaf power index, the raw Shapley-Shubik power index is not #P-parsimonious-complete.


Understanding the Paradoxical Effects of Power Control on the Capacity of Wireless Networks

Abstract: Recent works show conflicting results: network capacity may increase or decrease with higher transmission power under different scenarios. In this work, we want to understand this paradox. Specifically, we address the following questions: (1)Theoretically, should we increase or decrease transmission power to maximize network capacity? (2) Theoretically, how much network capacity gain can we achieve by power control? (3) Under realistic situations, how do power control, link scheduling and routing interact with each other? Under which scenarios can we expect a large capacity gain by using higher transmission power? To answer these questions, firstly, we prove that the optimal network capacity is a non-decreasing function of transmission power. Secondly, we prove that the optimal network capacity can be increased unlimitedly by higher transmission power in some network configurations. However, when nodes are distributed uniformly, the gain of optimal network capacity by higher transmission power is upper-bounded by a positive constant. Thirdly, we discuss why network capacity in practice may increase or decrease with higher transmission power under different scenarios using carrier sensing and the minimum hop-count routing. Extensive simulations are carried out to verify our analysis.


A Molecular Model for Communication through a Secrecy System

Abstract: Codes have been used for centuries to convey secret information.To a cryptanalyst, the interception of a code is only the first step in recovering a secret message.Deoxyribonucleic acid (DNA) is a biological and molecular code.Through the work of Marshall Nirenberg and others, DNA is now understood to specify for amino acids in triplet codes of bases.The possibilty of DNA encoding secret information in a natural language is explored, since a code is expected to have a distinct mathematical solution.


Breaking One-Round Key-Agreement Protocols in the Random Oracle Model

Abstract: In this paper we study one-round key-agreement protocols analogous to Merkle's puzzles in the random oracle model. The players Alice and Bob are allowed to query a random permutation oracle $n$ times and upon their queries and communication, they both output the same key with high probability. We prove that Eve can always break such a protocol by querying the oracle $O(n^2)$ times. The long-time unproven optimality of the quadratic bound in the fully general, multi-round scenario has been shown recently by Barak and Mahmoody-Ghidary. The results in this paper have been found independently of their work.


Methods to integrate a language model with semantic information for a word prediction component

Abstract: Most current word prediction systems make use of n-gram language models (LM) to estimate the probability of the following word in a phrase. In the past years there have been many attempts to enrich such language models with further syntactic or semantic information. We want to explore the predictive powers of Latent Semantic Analysis (LSA), a method that has been shown to provide reliable information on long-distance semantic dependencies between words in a context. We present and evaluate here several methods that integrate LSA-based information with a standard language model: a semantic cache, partial reranking, and different forms of interpolation. We found that all methods show significant improvements, compared to the 4-gram baseline, and most of them to a simple cache model as well.


Concerning Olga, the Beautiful Little Street Dancer (Adjectives as Higher-Order Polymorphic Functions)

Abstract: In this paper we suggest a typed compositional seman-tics for nominal compounds of the form [Adj Noun] that models adjectives as higher-order polymorphic functions, and where types are assumed to represent concepts in an ontology that reflects our commonsense view of the world and the way we talk about it in or-dinary language. In addition to [Adj Noun] compounds our proposal seems also to suggest a plausible explana-tion for well known adjective ordering restrictions.


Graceful Degradation of Air Traffic Operations

Abstract: The introduction of new technologies and concepts of operation in the air transportation system is not possible, unless they can be proven not to adversely affect the system operation under not only nominal, but also degraded conditions. In extreme scenarios, degraded operations due to partial or complete technological failures should never endanger system safety. Many past system evolutions, whether ground-based or airborne, have been based on trial-and-error, and system safety was addressed only after a specific event yielded dramatic or near- dramatic consequences. Future system evolutions, however, must leverage available computation, prior knowledge and abstract reasoning to anticipate all possible system degradations and prove that such degradations are graceful and safe. This paper is concerned with the graceful degradation of high-density, structured arrival traffic against partial or complete surveillance failures. It is shown that for equal performance requirements, some traffic configurations might be easier to handle than others, thereby offering a quantitative perspective on these traffic configurations. ability to "gracefully degrade". To support our work, we also introduce a new conflict resolution algorithm, aimed at solving conflicts involving many aircraft when aircraft position information is in the process of degrading.


Source Code Protection for Applications Written in Microsoft Excel and Google Spreadsheet

Abstract: Spreadsheets are used to develop application software that is distributed to users. Unfortunately, the users often have the ability to change the programming statements ("source code") of the spreadsheet application. This causes a host of problems. By critically examining the suitability of spreadsheet computer programming languages for application development, six "application development features" are identified, with source code protection being the most important. We investigate the status of these features and discuss how they might be implemented in the dominant Microsoft Excel spreadsheet and in the new Google Spreadsheet. Although Google Spreadsheet currently provides no source code control, its web-centric delivery model offers technical advantages for future provision of a rich set of features. Excel has a number of tools that can be combined to provide "pretty good protection" of source code, but weak passwords reduce its robustness. User access to Excel source code must be considered a programmer choice rather than an attribute of the spreadsheet.


Spreadsheet Assurance by "Control Around" is a Viable Alternative to the Traditional Approach

Abstract: The traditional approach to spreadsheet auditing generally consists of auditing every distinct formula within a spreadsheet. Although tools are developed to support auditors during this process, the approach is still very time consuming and therefore relatively expensive. As an alternative to the traditional "control through" approach, this paper discusses a "control around" approach. Within the proposed approach not all distinct formulas are audited separately, but the relationship between input data and output data of a spreadsheet is audited through comparison with a shadow model developed in a modelling language. Differences between the two models then imply possible errors in the spreadsheet. This paper describes relevant issues regarding the "control around" approach and the circumstances in which this approach is preferred above a traditional spreadsheet audit approach.


Non-Deterministic Communication Complexity of Regular Languages

Abstract: In this thesis, we study the place of regular languages within the communication complexity setting. In particular, we are interested in the non-deterministic communication complexity of regular languages. We show that a regular language has either O(1) or Omega(log n) non-deterministic complexity. We obtain several linear lower bound results which cover a wide range of regular languages having linear non-deterministic complexity. These lower bound results also imply a result in semigroup theory: we obtain sufficient conditions for not being in the positive variety Pol(Com). To obtain our results, we use algebraic techniques. In the study of regular languages, the algebraic point of view pioneered by Eilenberg (\cite{Eil74}) has led to many interesting results. Viewing a semigroup as a computational device that recognizes languages has proven to be prolific from both semigroup theory and formal languages perspectives. In this thesis, we provide further instances of such mutualism.


On the Complexity of Binary Samples

Abstract: Consider a class $\mH$ of binary functions $h: X\to\{-1, +1\}$ on a finite interval $X=[0, B]\subset \Real$. Define the {\em sample width} of $h$ on a finite subset (a sample) $S\subset X$ as $\w_S(h) \equiv \min_{x\in S} |\w_h(x)|$, where $\w_h(x) = h(x) \max\{a\geq 0: h(z)=h(x), x-a\leq z\leq x+a\}$. Let $\mathbb{S}_\ell$ be the space of all samples in $X$ of cardinality $\ell$ and consider sets of wide samples, i.e., {\em hypersets} which are defined as $A_{\beta, h} = \{S\in \mathbb{S}_\ell: \w_{S}(h) \geq \beta\}$. Through an application of the Sauer-Shelah result on the density of sets an upper estimate is obtained on the growth function (or trace) of the class $\{A_{\beta, h}: h\in\mH\}$, $\beta>0$, i.e., on the number of possible dichotomies obtained by intersecting all hypersets with a fixed collection of samples $S\in\mathbb{S}_\ell$ of cardinality $m$. The estimate is $2\sum_{i=0}^{2\lfloor B/(2\beta)\rfloor}{m-\ell\choose i}$.


Investigating the Potential of Test-Driven Development for Spreadsheet Engineering

Abstract: It is widely documented that the absence of a structured approach to spreadsheet engineering is a key factor in the high level of spreadsheet errors. In this paper we propose and investigate the application of Test-Driven Development to the creation of spreadsheets. Test-Driven Development is an emerging development technique in software engineering that has been shown to result in better quality software code. It has also been shown that this code requires less testing and is easier to maintain. Through a pair of case studies we demonstrate that Test-Driven Development can be applied to the development of spreadsheets. We present the detail of these studies preceded by a clear explanation of the technique and its application to spreadsheet engineering. A supporting tool under development by the authors is also documented along with proposed research to determine the effectiveness of the methodology and the associated tool.


Automatic Text Area Segmentation in Natural Images

Abstract: We present a hierarchical method for segmenting text areas in natural images. The method assumes that the text is written with a contrasting color on a more or less uniform background. But no assumption is made regarding the language or character set used to write the text. In particular, the text can contain simple graphics or symbols. The key feature of our approach is that we first concentrate on finding the background of the text, before testing whether there is actually text on the background. Since uniform areas are easy to find in natural images, and since text backgrounds define areas which contain "holes" (where the text is written) we thus look for uniform areas containing "holes" and label them as text backgrounds candidates. Each candidate area is then further tested for the presence of text within its convex hull. We tested our method on a database of 65 images including English and Urdu text. The method correctly segmented all the text areas in 63 of these images, and in only 4 of these were areas that do not contain text also segmented.


The REESSE2+ Public-key Encryption Scheme

Abstract: This paper gives the definitions of an anomalous super-increasing sequence and an anomalous subset sum separately, proves the two properties of an anomalous super-increasing sequence, and proposes the REESSE2+ public-key encryption scheme which includes the three algorithms for key generation, encryption and decryption. The paper discusses the necessity and sufficiency of the lever function for preventing the Shamir extremum attack, analyzes the security of REESSE2+ against extracting a private key from a public key through the exhaustive search, recovering a plaintext from a ciphertext plus a knapsack of high density through the L3 lattice basis reduction method, and heuristically obtaining a plaintext through the meet-in-the-middle attack or the adaptive-chosen-ciphertext attack. The authors evaluate the time complexity of REESSE2+ encryption and decryption algorithms, compare REESSE2+ with ECC and NTRU, and find that the encryption speed of REESSE2+ is ten thousand times faster than ECC and NTRU bearing the equivalent security, and the decryption speed of REESSE2+ is roughly equivalent to ECC and NTRU respectively.


Improved lower bound for deterministic broadcasting in radio networks

Abstract: We consider the problem of deterministic broadcasting in radio networks when the nodes have limited knowledge about the topology of the network. We show that for every deterministic broadcasting protocol there exists a network, of radius 2, for which the protocol takes at least $\Omega(\sqrt{n}) rounds for completing the broadcast. Our argument can be extended to prove a lower bound of Omega(\sqrt{nD}) rounds for broadcasting in radio networks of radius D. This resolves one of the open problems posed in [29], where in the authors proved a lower bound of $\Omega(n^{1/4}) rounds for broadcasting in constant diameter networks. We prove the new lower $\Omega(\sqrt{n})$ bound for a special family of radius 2 networks. Each network of this family consists of O(\sqrt{n}) components which are connected to each other via only the source node. At the heart of the proof is a novel simulation argument, which essentially says that any arbitrarily complicated strategy of the source node can be simulated by the nodes of the networks, if the source node just transmits partial topological knowledge about some component instead of arbitrary complicated messages. To the best of our knowledge this type of simulation argument is novel and may be useful in further improving the lower bound or may find use in other applications. Keywords: radio networks, deterministic broadcast, lower bound, advice string, simulation, selective families, limited topological knowledge.


Bicretieria Optimization in Routing Games

Abstract: Two important metrics for measuring the quality of routing paths are the maximum edge congestion $C$ and maximum path length $D$. Here, we study bicriteria in routing games where each player $i$ selfishly selects a path that simultaneously minimizes its maximum edge congestion $C_i$ and path length $D_i$. We study the stability and price of anarchy of two bicriteria games: - {\em Max games}, where the social cost is $\max(C,D)$ and the player cost is $\max(C_i, D_i)$. We prove that max games are stable and convergent under best-response dynamics, and that the price of anarchy is bounded above by the maximum path length in the players' strategy sets. We also show that this bound is tight in worst-case scenarios. - {\em Sum games}, where the social cost is $C+D$ and the player cost is $C_i+D_i$. For sum games, we first show the negative result that there are game instances that have no Nash-equilibria. Therefore, we examine an approximate game called the {\em sum-bucket game} that is always convergent (and therefore stable). We show that the price of anarchy in sum-bucket games is bounded above by $C^* \cdot D^* / (C^* + D^*)$ (with a poly-log factor), where $C^*$ and $D^*$ are the optimal coordinated congestion and path length. Thus, the sum-bucket game has typically superior price of anarchy bounds than the max game. In fact, when either $C^*$ or $D^*$ is small (e.g. constant) the social cost of the Nash-equilibria is very close to the coordinated optimal $C^* + D^*$ (within a poly-log factor). We also show that the price of anarchy bound is tight for cases where both $C^*$ and $D^*$ are large.


On the Double Coset Membership Problem for Permutation Groups

Abstract: We show that the Double Coset Membership problem for permutation groups possesses perfect zero-knowledge proofs.


Zero-Knowledge Proofs of the Conjugacy for Permutation Groups

Abstract: We design a perfect zero-knowledge proof system for recognition if two permutation groups are conjugate.


Improved Deterministic Length Reduction

Abstract: This paper presents a new technique for deterministic length reduction. This technique improves the running time of the algorithm presented in \cite{LR07} for performing fast convolution in sparse data. While the regular fast convolution of vectors $V_1,V_2$ whose sizes are $N_1,N_2$ respectively, takes $O(N_1 \log N_2)$ using FFT, using the new technique for length reduction, the algorithm proposed in \cite{LR07} performs the convolution in $O(n_1 \log^3 n_1)$, where $n_1$ is the number of non-zero values in $V_1$. The algorithm assumes that $V_1$ is given in advance, and $V_2$ is given in running time. The novel technique presented in this paper improves the convolution time to $O(n_1 \log^2 n_1)$ {\sl deterministically}, which equals the best running time given achieved by a {\sl randomized} algorithm. The preprocessing time of the new technique remains the same as the preprocessing time of \cite{LR07}, which is $O(n_1^2)$. This assumes and deals the case where $N_1$ is polynomial in $n_1$. In the case where $N_1$ is exponential in $n_1$, a reduction to a polynomial case can be used. In this paper we also improve the preprocessing time of this reduction from $O(n_1^4)$ to $O(n_1^3{\rm polylog}(n_1))$.


Solving the Maximum Agreement SubTree and the Maximum Compatible Tree problems on many bounded degree trees

Abstract: Given a set of leaf-labeled trees with identical leaf sets, the well-known "Maximum Agreement SubTree" problem (MAST) consists of finding a subtree homeomorphically included in all input trees and with the largest number of leaves. Its variant called "Maximum Compatible Tree" (MCT) is less stringent, as it allows the input trees to be refined. Both problems are of particular interest in computational biology, where trees encountered have often small degrees. In this paper, we study the parameterized complexity of MAST and MCT with respect to the maximum degree, denoted by D, of the input trees. It is known that MAST is polynomial for bounded D. As a counterpart, we show that the problem is W[1]-hard with respect to parameter D. Moreover, relying on recent advances in parameterized complexity we obtain a tight lower bound: while MAST can be solved in O(N^{O(D)}) time where N denotes the input length, we show that an O(N^{o(D)}) bound is not achievable, unless SNP is contained in SE. We also show that MCT is W[1]-hard with respect to D, and that MCT cannot be solved in O(N^{o(2^{D/2})}) time, SNP is contained in SE.


Shallow Models for Non-Iterative Modal Logics

Abstract: The methods used to establish PSPACE-bounds for modal logics can roughly be grouped into two classes: syntax driven methods establish that exhaustive proof search can be performed in polynomial space whereas semantic approaches directly construct shallow models. In this paper, we follow the latter approach and establish generic PSPACE-bounds for a large and heterogeneous class of modal logics in a coalgebraic framework. In particular, no complete axiomatisation of the logic under scrutiny is needed. This does not only complement our earlier, syntactic, approach conceptually, but also covers a wide variety of new examples which are difficult to harness by purely syntactic means. Apart from re-proving known complexity bounds for a large variety of structurally different logics, we apply our method to obtain previously unknown PSPACE-bounds for Elgesem's logic of agency and for graded modal logic over reflexive frames.


Fault-Tolerant Partial Replication in Large-Scale Database Systems

Abstract: We investigate a decentralised approach to committing transactions in a replicated database, under partial replication. Previous protocols either re-execute transactions entirely and/or compute a total order of transactions. In contrast, ours applies update values, and orders only conflicting transactions. It results that transactions execute faster, and distributed databases commit in small committees. Both effects contribute to preserve scalability as the number of databases and transactions increase. Our algorithm ensures serializability, and is live and safe in spite of faults.


Partitioning the Threads of a Mobile System

Abstract: In this paper, we show how thread partitioning helps in proving properties of mobile systems. Thread partitioning consists in gathering the threads of a mobile system into several classes. The partitioning criterion is left as a parameter of both the mobility model and the properties we are interested in. Then, we design a polynomial time abstract interpretation-based static analysis that counts the number of threads inside each partition class.


A topological formal treatment for scenario-based software specification of concurrent real-time systems

Abstract: Real-time systems are computing systems in which the meeting of their requirements is vital for their correctness. Consequently, if the real-time requirements of these systems are poorly understood and verified, the results can be disastrous and lead to irremediable project failures at the early phases of development. The present work addresses the problem of detecting deadlock situations early in the requirements specification phase of a concurrent real time system, proposing a simple proof-of-concepts prototype that joins scenario-based requirements specifications and techniques based on topology. The efforts are concentrated in the integration of the formal representation of Message Sequence Chart scenarios into the deadlock detection algorithm of Fajstrup et al., based on geometric and algebraic topology.


Multi-Layer Perceptrons and Symbolic Data

Abstract: In some real world situations, linear models are not sufficient to represent accurately complex relations between input variables and output variables of a studied system. Multilayer Perceptrons are one of the most successful non-linear regression tool but they are unfortunately restricted to inputs and outputs that belong to a normed vector space. In this chapter, we propose a general recoding method that allows to use symbolic data both as inputs and outputs to Multilayer Perceptrons. The recoding is quite simple to implement and yet provides a flexible framework that allows to deal with almost all practical cases. The proposed method is illustrated on a real world data set.


Acc\'el\'eration des cartes auto-organisatrices sur tableau de dissimilarit\'es par s\'eparation et \'evaluation

Abstract: In this paper, a new implementation of the adaptation of Kohonen self-organising maps (SOM) to dissimilarity matrices is proposed. This implementation relies on the branch and bound principle to reduce the algorithm running time. An important property of this new approach is that the obtained algorithm produces exactly the same results as the standard algorithm.


A proposal to a generalised splicing with a self assembly approach

Abstract: Theory of splicing is an abstract model of the recombinant behaviour of DNAs. In a splicing system, two strings to be spliced are taken from the same set and the splicing rule is from another set. Here we propose a generalised splicing (GS) model with three components, two strings from two languages and a splicing rule from third component. We propose a generalised self assembly (GSA) of strings. Two strings $u_1xv_1$ and $u_2xv_2$ self assemble over $x$ and generate $u_1xv_2$ and $u_2xv_1$. We study the relationship between GS and GSA. We study some classes of generalised splicing languages with the help of generalised self assembly.


A data-driven functional projection approach for the selection of feature ranges in spectra with ICA or cluster analysis

Abstract: Prediction problems from spectra are largely encountered in chemometry. In addition to accurate predictions, it is often needed to extract information about which wavelengths in the spectra contribute in an effective way to the quality of the prediction. This implies to select wavelengths (or wavelength intervals), a problem associated to variable selection. In this paper, it is shown how this problem may be tackled in the specific case of smooth (for example infrared) spectra. The functional character of the spectra (their smoothness) is taken into account through a functional variable projection procedure. Contrarily to standard approaches, the projection is performed on a basis that is driven by the spectra themselves, in order to best fit their characteristics. The methodology is illustrated by two examples of functional projection, using Independent Component Analysis and functional variable clustering, respectively. The performances on two standard infrared spectra benchmarks are illustrated.


On the complexity of finding gapped motifs

Abstract: This paper has been withdrawn by the corresponding author because the newest version is now published in Journal of Discrete Algorithms.


Approximability Distance in the Space of H-Colourability Problems

Abstract: A graph homomorphism is a vertex map which carries edges from a source graph to edges in a target graph. We study the approximability properties of the Weighted Maximum H-Colourable Subgraph problem (MAX H-COL). The instances of this problem are edge-weighted graphs G and the objective is to find a subgraph of G that has maximal total edge weight, under the condition that the subgraph has a homomorphism to H; note that for H=K_k this problem is equivalent to MAX k-CUT. To this end, we introduce a metric structure on the space of graphs which allows us to extend previously known approximability results to larger classes of graphs. Specifically, the approximation algorithms for MAX CUT by Goemans and Williamson and MAX k-CUT by Frieze and Jerrum can be used to yield non-trivial approximation results for MAX H-COL. For a variety of graphs, we show near-optimality results under the Unique Games Conjecture. We also use our method for comparing the performance of Frieze & Jerrum's algorithm with Hastad's approximation algorithm for general MAX 2-CSP. This comparison is, in most cases, favourable to Frieze & Jerrum.


Popularity, Novelty and Attention

Abstract: We analyze the role that popularity and novelty play in attracting the attention of users to dynamic websites. We do so by determining the performance of three different strategies that can be utilized to maximize attention. The first one prioritizes novelty while the second emphasizes popularity. A third strategy looks myopically into the future and prioritizes stories that are expected to generate the most clicks within the next few minutes. We show that the first two strategies should be selected on the basis of the rate of novelty decay, while the third strategy performs sub-optimally in most cases. We also demonstrate that the relative performance of the first two strategies as a function of the rate of novelty decay changes abruptly around a critical value, resembling a phase transition in the physical world. 1


Improving Performance of Cluster Based Routing Protocol using Cross-Layer Design

Abstract: The main goal of routing protocol is to efficiency delivers data from source to destination. All routing protocols are the same in this goal, but the way they adopt to achieve it is different, so routing strategy has an egregious role on the performance of an ad hoc network. Most of routing protocols proposed for ad hoc networks have a flat structure. These protocols expand the control overhead packets to discover or maintain a route. On the other hand a number of hierarchical-based routing protocols have been developed, mostly are based on layered design. These protocols improve network performances especially when the network size grows up since details about remote portion of network can be handled in an aggregate manner. Although, there is another approach to design a protocol called cross-layer design. Using this approach information can exchange between different layer of protocol stack, result in optimizing network performances. In this paper, we intend to exert cross-layer design to optimize Cluster Based Routing Protocol (Cross-CBRP). Using NS-2 network simulator we evaluate rate of cluster head changes, throughput and packet delivery ratio. Comparisons denote that Cross-CBRP has better performances with respect to the original CBRP.


Energy Aware Self-Organizing Density Management in Wireless Sensor Networks

Abstract: Energy consumption is the most important factor that determines sensor node lifetime. The optimization of wireless sensor network lifetime targets not only the reduction of energy consumption of a single sensor node but also the extension of the entire network lifetime. We propose a simple and adaptive energy-conserving topology management scheme, called SAND (Self-Organizing Active Node Density). SAND is fully decentralized and relies on a distributed probing approach and on the redundancy resolution of sensors for energy optimizations, while preserving the data forwarding and sensing capabilities of the network. We present the SAND's algorithm, its analysis of convergence, and simulation results. Simulation results show that, though slightly increasing path lengths from sensor to sink nodes, the proposed scheme improves significantly the network lifetime for different neighborhood densities degrees, while preserving both sensing and routing fidelity.


Timed Quorum System for Large-Scale and Dynamic Environments

Abstract: This paper presents Timed Quorum System (TQS), a new quorum system especially suited for large-scale and dynamic systems. TQS requires that two quorums intersect with high probability if they are used in the same small period of time. It proposed an algorithm that implements TQS and that verifies probabilistic atomicity: a consistency criterion that requires each operation to respect atomicity with high probability. This TQS implementation has quorum of size O(\sqrt{nD}) and expected access time of O(log \sqrt{nD}) message delays, where n measures the size of the system and D is a required parameter to handle dynamism.


Trusted-HB: a low-cost version of HB+ secure against Man-in-The-Middle attacks

Abstract: Since the introduction at Crypto'05 by Juels and Weis of the protocol HB+, a lightweight protocol secure against active attacks but only in a detection based-model, many works have tried to enhance its security. We propose here a new approach to achieve resistance against Man-in-The-Middle attacks. Our requirements - in terms of extra communications and hardware - are surprisingly low.


(Generalized) Post Correspondence Problem and semi-Thue systems

Abstract: Let PCP(k) denote the Post Correspondence Problem for k input pairs of strings. Let ACCESSIBILITY(k) denote the the word problem for k-rule semi-Thue systems. In 1980, Claus showed that if ACCESSIBILITY(k) is undecidable then PCP(k + 4) is also undecidable. The aim of the paper is to present a clean, detailed proof of the statement. We proceed in two steps, using the Generalized Post Correspondence Problem as an auxiliary. First, we prove that if ACCESSIBILITY(k) is undecidable then GPCP(k + 2) is also undecidable. Then, we prove that if GPCP(k) is undecidable then PCP(k + 2) is also undecidable. (The latter result has also been shown by Harju and Karhumaki.) To date, the sharpest undecidability bounds for both PCP and GPCP have been deduced from Claus's result: since Matiyasevich and Senizergues showed that ACCESSIBILITY(3) is undecidable, GPCP(5) and PCP(7) are undecidable.


Knowledge management by wikis

Abstract: Wikis provide a new way of collaboration and knowledge sharing. Wikis are software that allows users to work collectively on a web-based knowledge base. Wikis are characterised by a sense of anarchism, collaboration, connectivity, organic development and self-healing, and they rely on trust. We list several concerns about applying wikis in professional organisation. After these concerns are met, wikis can provide a progessive, new knowledge sharing and collaboration tool.


Modelling and Analysis of the Distributed Coordination Function of IEEE 802.11 with Multirate Capability

Abstract: The aim of this paper is twofold. On one hand, it presents a multi-dimensional Markovian state transition model characterizing the behavior at the Medium Access Control (MAC) layer by including transmission states that account for packet transmission failures due to errors caused by propagation through the channel, along with a state characterizing the system when there are no packets to be transmitted in the queue of a station (to model non-saturated traffic conditions). On the other hand, it provides a throughput analysis of the IEEE 802.11 protocol at the data link layer in both saturated and non-saturated traffic conditions taking into account the impact of both transmission channel and multirate transmission in Rayleigh fading environment. Simulation results closely match the theoretical derivations confirming the effectiveness of the proposed model.


D\'eveloppement et analyse multi outils d'un protocole MAC d\'eterministe pour un r\'eseau de capteurs sans fil

Abstract: In this article, we present a multi-tool method for the development and the analysis of a new medium access method. IEEE 802.15.4 / ZigBee technology has been used as a basis for this new determinist MAC layer which enables a high level of QoS. This WPAN can be typically used for wireless sensor networks which require strong temporal constraints. To validate the proposed protocol, three complementary and adequate tools are used: Petri Nets for the formal validation of the algorithm, a dedicated simulator for the temporal aspects, and some measures on a real prototype based on a couple of ZigBee FREESCALE components for the hardware characterization of layers #1 and #2.


Independence and concurrent separation logic

Abstract: A compositional Petri net-based semantics is given to a simple language allowing pointer manipulation and parallelism. The model is then applied to give a notion of validity to the judgements made by concurrent separation logic that emphasizes the process-environment duality inherent in such rely-guarantee reasoning. Soundness of the rules of concurrent separation logic with respect to this definition of validity is shown. The independence information retained by the Petri net model is then exploited to characterize the independence of parallel processes enforced by the logic. This is shown to permit a refinement operation capable of changing the granularity of atomic actions.


Distributed Double Spending Prevention

Abstract: We study the problem of preventing double spending in electronic payment schemes in a distributed fashion. This problem occurs, for instance, when the spending of electronic coins needs to be controlled by a large collection of nodes (eg. in a peer-to-peer (P2P) system) instead of one central bank. Contrary to the commonly held belief that this is fundamentally impossible, we propose several solutions that do achieve a reasonable level of double spending prevention, and analyse their efficiency under varying assumptions.


The Ephemeral Pairing Problem

Abstract: In wireless ad-hoc broadcast networks the pairing problem consists of establishing a (long-term) connection between two specific physical nodes in the network that do not yet know each other. We focus on the ephemeral version of this problem. Ephemeral pairings occur, for example, when electronic business cards are exchanged between two people that meet, or when one pays at a check-out using a wireless wallet. This problem can, in more abstract terms, be phrased as an ephemeral key exchange problem: given a low bandwidth authentic (or private) communication channel between two nodes, and a high bandwidth broadcast channel, can we establish a high-entropy shared secret session key between the two nodes without relying on any a priori shared secret information. Apart from introducing this new problem, we present several ephemeral key exchange protocols, both for the case of authentic channels as well as for the case of private channels.


Using Bayesian Blocks to Partition Self-Organizing Maps

Abstract: Self organizing maps (SOMs) are widely-used for unsupervised classification. For this application, they must be combined with some partitioning scheme that can identify boundaries between distinct regions in the maps they produce. We discuss a novel partitioning scheme for SOMs based on the Bayesian Blocks segmentation algorithm of Scargle [1998]. This algorithm minimizes a cost function to identify contiguous regions over which the values of the attributes can be represented as approximately constant. Because this cost function is well-defined and largely independent of assumptions regarding the number and structure of clusters in the original sample space, this partitioning scheme offers significant advantages over many conventional methods. Sample code is available.


Combining generic judgments with recursive definitions

Abstract: Many semantical aspects of programming languages, such as their operational semantics and their type assignment calculi, are specified by describing appropriate proof systems. Recent research has identified two proof-theoretic features that allow direct, logic-based reasoning about such descriptions: the treatment of atomic judgments as fixed points (recursive definitions) and an encoding of binding constructs via generic judgments. However, the logics encompassing these two features have thus far treated them orthogonally: that is, they do not provide the ability to define object-logic properties that themselves depend on an intrinsic treatment of binding. We propose a new and simple integration of these features within an intuitionistic logic enhanced with induction over natural numbers and we show that the resulting logic is consistent. The pivotal benefit of the integration is that it allows recursive definitions to not just encode simple, traditional forms of atomic judgments but also to capture generic properties pertaining to such judgments. The usefulness of this logic is illustrated by showing how it can provide elegant treatments of object-logic contexts that appear in proofs involving typing calculi and of arbitrarily cascading substitutions that play a role in reducibility arguments.


New Estimation Procedures for PLS Path Modelling

Abstract: Given R groups of numerical variables X1, ... XR, we assume that each group is the result of one underlying latent variable, and that all latent variables are bound together through a linear equation system. Moreover, we assume that some explanatory latent variables may interact pairwise in one or more equations. We basically consider PLS Path Modelling's algorithm to estimate both latent variables and the model's coefficients. New "external" estimation schemes are proposed that draw latent variables towards strong group structures in a more flexible way. New "internal" estimation schemes are proposed to enable PLSPM to make good use of variable group complementarity and to deal with interactions. Application examples are given.


Small Is Not Always Beautiful

Abstract: Peer-to-peer content distribution systems have been enjoying great popularity, and are now gaining momentum as a means of disseminating video streams over the Internet. In many of these protocols, including the popular BitTorrent, content is split into mostly fixed-size pieces, allowing a client to download data from many peers simultaneously. This makes piece size potentially critical for performance. However, previous research efforts have largely overlooked this parameter, opting to focus on others instead. This paper presents the results of real experiments with varying piece sizes on a controlled BitTorrent testbed. We demonstrate that this parameter is indeed critical, as it determines the degree of parallelism in the system, and we investigate optimal piece sizes for distributing small and large content. We also pinpoint a related design trade-off, and explain how BitTorrent's choice of dividing pieces into subpieces attempts to address it.


An Empirical Study of Cache-Oblivious Priority Queues and their Application to the Shortest Path Problem

Abstract: In recent years the Cache-Oblivious model of external memory computation has provided an attractive theoretical basis for the analysis of algorithms on massive datasets. Much progress has been made in discovering algorithms that are asymptotically optimal or near optimal. However, to date there are still relatively few successful experimental studies. In this paper we compare two different Cache-Oblivious priority queues based on the Funnel and Bucket Heap and apply them to the single source shortest path problem on graphs with positive edge weights. Our results show that when RAM is limited and data is swapping to external storage, the Cache-Oblivious priority queues achieve orders of magnitude speedups over standard internal memory techniques. However, for the single source shortest path problem both on simulated and real world graph data, these speedups are markedly lower due to the time required to access the graph adjacency list itself.


Average-Case Analysis of Online Topological Ordering

Abstract: Many applications like pointer analysis and incremental compilation require maintaining a topological ordering of the nodes of a directed acyclic graph (DAG) under dynamic updates. All known algorithms for this problem are either only analyzed for worst-case insertion sequences or only evaluated experimentally on random DAGs. We present the first average-case analysis of online topological ordering algorithms. We prove an expected runtime of O(n^2 polylog(n)) under insertion of the edges of a complete DAG in a random order for the algorithms of Alpern et al. (SODA, 1990), Katriel and Bodlaender (TALG, 2006), and Pearce and Kelly (JEA, 2006). This is much less than the best known worst-case bound O(n^{2.75}) for this problem.


New Extensions of Pairing-based Signatures into Universal (Multi) Designated Verifier Signatures

Abstract: The concept of universal designated verifier signatures was introduced by Steinfeld, Bull, Wang and Pieprzyk at Asiacrypt 2003. These signatures can be used as standard publicly verifiable digital signatures but have an additional functionality which allows any holder of a signature to designate the signature to any desired verifier. This designated verifier can check that the message was indeed signed, but is unable to convince anyone else of this fact. We propose new efficient constructions for pairing-based short signatures. Our first scheme is based on Boneh-Boyen signatures and its security can be analyzed in the standard security model. We prove its resistance to forgery assuming the hardness of the so-called strong Diffie-Hellman problem, under the knowledge-of-exponent assumption. The second scheme is compatible with the Boneh-Lynn-Shacham signatures and is proven unforgeable, in the random oracle model, under the assumption that the computational bilinear Diffie-Hellman problem is untractable. Both schemes are designed for devices with constrained computation capabilities since the signing and the designation procedure are pairing-free. Finally, we present extensions of these schemes in the multi-user setting proposed by Desmedt in 2003.


Multi-Use Unidirectional Proxy Re-Signatures

Abstract: In 1998, Blaze, Bleumer, and Strauss suggested a cryptographic primitive named proxy re-signatures where a proxy turns a signature computed under Alice's secret key into one from Bob on the same message. The semi-trusted proxy does not learn either party's signing key and cannot sign arbitrary messages on behalf of Alice or Bob. At CCS 2005, Ateniese and Hohenberger revisited the primitive by providing appropriate security definitions and efficient constructions in the random oracle model. Nonetheless, they left open the problem of designing a multi-use unidirectional scheme where the proxy is able to translate in only one direction and signatures can be re-translated several times. This paper solves this problem, suggested for the first time 10 years ago, and shows the first multi-hop unidirectional proxy re-signature schemes. We describe a random-oracle-using system that is secure in the Ateniese-Hohenberger model. The same technique also yields a similar construction in the standard model (i.e. without relying on random oracles). Both schemes are efficient and require newly defined -- but falsifiable -- Diffie-Hellman-like assumptions in bilinear groups.


Snap-Stabilization in Message-Passing Systems

Abstract: In this paper, we tackle the open problem of snap-stabilization in message-passing systems. Snap-stabilization is a nice approach to design protocols that withstand transient faults. Compared to the well-known self-stabilizing approach, snap-stabilization guarantees that the effect of faults is contained immediately after faults cease to occur. Our contribution is twofold: we show that (1) snap-stabilization is impossible for a wide class of problems if we consider networks with finite yet unbounded channel capacity; (2) snap-stabilization becomes possible in the same setting if we assume bounded-capacity channels. We propose three snap-stabilizing protocols working in fully-connected networks. Our work opens exciting new research perspectives, as it enables the snap-stabilizing paradigm to be implemented in actual networks.


Note sur les temps de service r\'esiduels dans les syst\`emes type M/G/c

Abstract: Approximations for the mean performance indices for the M/G/c queue rely on the approximate computation of the probability that an arriving request has to wait for service and of the minimum of residual service times if all servers are found busy. Using numerical examples, we investigate properties of these two quantities. In particular, we show that the minimum of residual service times depends on higher order properties, beyond the first two moments, of the service time distribution. Improved knowledge of the properties of the two quantities studied in this paper provides insight into avenues for improving the accuracy of approximations for the M/G/c queue.


Lower Bounds for Complementation of omega-Automata Via the Full Automata Technique

Abstract: In this paper, we first introduce a lower bound technique for the state complexity of transformations of automata. Namely we suggest first considering the class of full automata in lower bound analysis, and later reducing the size of the large alphabet via alphabet substitutions. Then we apply such technique to the complementation of nondeterministic \omega-automata, and obtain several lower bound results. Particularly, we prove an \omega((0.76n)^n) lower bound for B\"uchi complementation, which also holds for almost every complementation or determinization transformation of nondeterministic omega-automata, and prove an optimal (\omega(nk))^n lower bound for the complementation of generalized B\"uchi automata, which holds for Streett automata as well.


Minimum Entropy Orientations

Abstract: We study graph orientations that minimize the entropy of the in-degree sequence. The problem of finding such an orientation is an interesting special case of the minimum entropy set cover problem previously studied by Halperin and Karp [Theoret. Comput. Sci., 2005] and by the current authors [Algorithmica, to appear]. We prove that the minimum entropy orientation problem is NP-hard even if the graph is planar, and that there exists a simple linear-time algorithm that returns an approximate solution with an additive error guarantee of 1 bit. This improves on the only previously known algorithm which has an additive error guarantee of log_2 e bits (approx. 1.4427 bits).


Bayesian Nonlinear Principal Component Analysis Using Random Fields

Abstract: We propose a novel model for nonlinear dimension reduction motivated by the probabilistic formulation of principal component analysis. Nonlinearity is achieved by specifying different transformation matrices at different locations of the latent space and smoothing the transformation using a Markov random field type prior. The computation is made feasible by the recent advances in sampling from von Mises-Fisher distributions.


Untangling polygons and graphs

Abstract: Untangling is a process in which some vertices of a planar graph are moved to obtain a straight-line plane drawing. The aim is to move as few vertices as possible. We present an algorithm that untangles the cycle graph C_n while keeping at least \Omega(n^{2/3}) vertices fixed. For any graph G, we also present an upper bound on the number of fixed vertices in the worst case. The bound is a function of the number of vertices, maximum degree and diameter of G. One of its consequences is the upper bound O((n log n)^{2/3}) for all 3-vertex-connected planar graphs.


Some results on (a:b)-choosability

Abstract: A solution to a problem of Erd\H{o}s, Rubin and Taylor is obtained by showing that if a graph $G$ is $(a:b)$-choosable, and $c/d > a/b$, then $G$ is not necessarily $(c:d)$-choosable. Applying probabilistic methods, an upper bound for the $k^{th}$ choice number of a graph is given. We also prove that a directed graph with maximum outdegree $d$ and no odd directed cycle is $(k(d+1):k)$-choosable for every $k \geq 1$. Other results presented in this article are related to the strong choice number of graphs (a generalization of the strong chromatic number). We conclude with complexity analysis of some decision problems related to graph choosability.


Guarding curvilinear art galleries with edge or mobile guards via 2-dominance of triangulation graphs

Abstract: We consider the problem of monitoring an art gallery modeled as a polygon, the edges of which are arcs of curves, with edge or mobile guards. Our focus is on piecewise-convex polygons, i.e., polygons that are locally convex, except possibly at the vertices, and their edges are convex arcs. We transform the problem of monitoring a piecewise-convex polygon to the problem of 2-dominating a properly defined triangulation graph with edges or diagonals, where 2-dominance requires that every triangle in the triangulation graph has at least two of its vertices in its 2-dominating set. We show that $\lfloor\frac{n+1}{3}\rfloor$ diagonal guards or $\lfloor\frac{2n+1}{5}\rfloor$ edge guards are always sufficient and sometimes necessary, in order to 2-dominate a triangulation graph. Furthermore, we show how to compute: a diagonal 2-dominating set of size $\lfloor\frac{n+1}{3}\rfloor$ in linear time, an edge 2-dominating set of size $\lfloor\frac{2n+1}{5}\rfloor$ in $O(n^2)$ time, and an edge 2-dominating set of size $\lfloor\frac{3n}{7}\rfloor$ in O(n) time. Based on the above-mentioned results, we prove that, for piecewise-convex polygons, we can compute: a mobile guard set of size $\lfloor\frac{n+1}{3}\rfloor$ in $O(n\log{}n)$ time, an edge guard set of size $\lfloor\frac{2n+1}{5}\rfloor$ in $O(n^2)$ time, and an edge guard set of size $\lfloor\frac{3n}{7}\rfloor$ in $O(n\log{}n)$ time. Finally, we show that $\lfloor\frac{n}{3}\rfloor$ mobile or $\lceil\frac{n}{3}\rceil$ edge guards are sometimes necessary. When restricting our attention to monotone piecewise-convex polygons, the bounds mentioned above drop: $\lceil\frac{n+1}{4}\rceil$ edge or mobile guards are always sufficient and sometimes necessary; such an edge or mobile guard set, of size at most $\lceil\frac{n+1}{4}\rceil$, can be computed in O(n) time.