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In this paper, a self-stabilizing algorithm is presented for finding the articulation points of a connected undirected graph on a distributed or network model of computation after O(n[SUP2]|E|) moves. The algorithm is resilient to transient faults and does not require initialization. A correctness proof of the algorithm is also presented. The paper concludes with remarks on issues such as the time complexity of the algorithm and open problems. [ABSTRACT FROM AUTHOR]

This paper presents a new lower bound for the model-checking algorithm based on solving parity games due to Stevens and Stirling. We outline a family of games of linear size on which the algorithm requires exponential time. [ABSTRACT FROM AUTHOR]

The leader election problem is unsolvable for some anonymous networks. A leader election algorithm for anonymous networks thus elects a leader whenever it is possible; if it is impossible, the algorithm reports this fact. This paper investigates the space complexity of the leader election problem in anonymous networks, where the space complexity is measured by the size (in the number of bits) of memory per processor used by a leader election algorithm. We first observe that Ω(M + log d) bits are necessary and then show that O(n log d) bits are sufficient to construct a leader election algorithm that works on any network, where n, d and M are the number of processors, the maximum number of adjacent processors, and the maximum size (in bits) of a message, respectively. We next show that, for any arbitrarily fixed constant n, O(1) bits are sufficient to construct a leader election algorithm that works in any network of size n. [ABSTRACT FROM AUTHOR]

For a string w ∈ {0,1, 2,..., d-1}*, let vald(w) denote the integer whose base d representation is the string w and let MSDd(x) denote the most significant or the leading digit of a positive integer x when x is written as a base d integer. Hirvensalo and Karhumäki [9] studied the problem of computing the leading digit in the ternary representation of 2x ans showed that this problem has a polynomial time algorithm. In [16], some applications are presented for the problem of computing the leading digit of AB for given inputs A and B. In this paper, we study this problem from a formal language point of view. Formally, we consider the language Lb,d,j = {w|w ∈ {0,1, 2,..., d-1}*, 1 ≤ j ≤ 9, MSDb(dvalb(w))) = j} (and some related classes of languages) and address the question of whether this and some related languages are context-free. Standard pumping lemma proofs seem to be very difficult for these languages. We present a unified and simple combinatorial technique that shows that these languages are not unambiguous context-free languages. The Benford-Newcomb distribution plays a central role in our proofs. [ABSTRACT FROM AUTHOR]

As is well-known, Axel Thue constructed an infinite word over a 3-letter alphabet that contains no squares, that is, no nonempty subwords of the form xx. In this paper we consider a variation on this problem, where we try to avoid approximate squares, that is, subwords of the form xx' where |x| = |x'| and x and x' are "nearly" identical. [ABSTRACT FROM AUTHOR]

Sense of direction is a property of the edge-labeling of a network whose availability facilitates computations and often decreases their complexity. In this paper we consider the problem of providing a sense of direction to an anonymous, unoriented torus. The edges of the torus are initially labeled arbitrarily, and they have to be relabeled with a classical compass sense of direction. We first describe an algorithm where the relabeling of the edges is performed by a mobile agent that carries a token. The algorithm is optimal both in terms of number of tokens (with no tokens the task cannot be performed), and in terms of number of moves. We then show that the same technique can be used to orient the torus in the classical message-passing environment, thus obtaining an orientation algorithm that improves all the existing ones in this setting. [ABSTRACT FROM AUTHOR]

In this paper we consider a network where an intruder is moving "contaminating" the nodes it passes by, and we focus on the problem of decontaminating such a network by a team of mobile agents. The contamination/decontamination process has the following asynchronous dynamics: when the team is deployed all nodes are assumed to be contaminated, when an agent transits on a node, it will clean the node, when the node is left with no agent, the node will be recontaminated as soon as at least one of its neighbours is contaminated. We study the problem in asynchronous chordal ring networks and in tori. We consider two variations of the model: one where agents have only local knowledge, the other in which they have "visibility", i.e., they can "see" the state of their neighbouring nodes. We first derive lower bounds on the minimum number of agents necessary for the decontamination. In the case of chordal rings we show that the number of agents necessary to perform the cleaning does not depend on the size of the network; in fact it is linear in the length of the longest chord (provided that it is not too long). In the case of a torus, the minimum number of agents is equal to 2 · h, where h is the smallest dimension. We then propose optimal strategies for decontamination and we analyse the number of moves and the time complexity of the decontamination algorithms, showing that the visibility assumption allows us to decrease substantially both complexity measures. Another advantage of the "visibility model" is that agents move independently and autonomously without requiring any coordination. [ABSTRACT FROM AUTHOR]

We consider the following online scheduling problem. We are given a set of jobs, each having an integral release time and deadline, unit processing length, and a nonnegative real weight. In each time unit one job is to be scheduled, and the objective is to maximize the total value (weight) obtained by scheduling the jobs. This problem arises in the scheduling of packets in network switches supporting quality-of-service (QoS). Previous algorithms for this problem are 2-competitive. in this paper we propose a new algorithm that achieves an improved competitive ratio when the importance ratio is bounded. Specifically, for job weights within the range [1..B], our algorithm is 2 - 1/([lg B] + 2)-competitive, and the bound is tight. [ABSTRACT FROM AUTHOR]

The subtree transfer (STT) distance is one of the distance metric for comparing phylogenies. Previous work on computing the STT distance considered degree-3 trees only. In this paper, we give an approximation algorithm for the STT distance for degree-d trees with arbitrary d and with generalized STT operations. Also, some NP-hardness results related to STT distance are presented. [ABSTRACT FROM AUTHOR]

Hybrid dynamic systems include both continuous and discrete state variables. Properties of hybrid systems, which have an infinite state space, can often be verified using ordinary model checking together with a finite-state abstraction. Model checking can be inconclusive, however, in which case the abstraction must be refined. This paper presents a new procedure to perform this refinement operation for abstractions of hybrid systems. Following an approach originally developed for finite-state systems [11, 25], the refinement procedure constructs a new abstraction that eliminates a counterexample generated by the model checker. For hybrid systems, analysis of the counterexample requires the computation of sets of reachable states in the continuous state space. We show how such reachability computations with varying degrees of complexity can be used to refine hybrid system abstractions efficiently. Examples illustrate our counterexample-guided refinement procedure. Experimental results for a prototype implementation indicate significant advantages over existing methods. [ABSTRACT FROM AUTHOR]

In this paper we discuss efficient symbolic representations for infinite-state systems specified using linear arithmetic constraints. We give algorithms for constructing finite automata which represent integer sets that satisfy linear constraints. These automata can represent either signed or unsigned integers and have a lower number of states compared to other similar approaches. We present efficient storage techniques for the transition function of the automata and extend the construction algorithms to formulas on both boolean and integer variables. We also derive conditions which guarantee that the pre-condition computations used in symbolic verification algorithms do not cause an exponential increase in the automata size. We experimentally compare different symbolic representations by using them to verify non-trivial concurrent systems. Experimental results show that the symbolic representations based on our construction algorithms outperform the polyhedral representation used in Omega Library, and the automata representation used in LASH. [ABSTRACT FROM AUTHOR]