Showing papers by "Giuseppe Prencipe published in 2002"
01 Jan 2002
TL;DR: This work introduces a model of weak robots which can observe the set of all points in the plane which are occupied by other robots and strengthens them with the ability to detect whether more than one robot is at a point (multiplicity).
Abstract: We study the problem of coordinating a set of autonomous mobile robots that can freely move in a two-dimensional plane; in particular, we want them to gather at a point not fixed in advance (GATHERING PROBLEM). We introduce a model of weak robots (decentralized, asynchronous, no common knowledge, no identities, no central coordination, no direct communication, oblivious) which can observe the set of all points in the plane which are occupied by other robots. Based on this observation, a robot uses a deterministic algorithm to compute a destination, and moves there. We prove that these robots are too weak to gather at a point in finite time. Therefore, we strengthen them with the ability to detect whether more than one robot is at a point (multiplicity). We analyze the GATHERING PROBLEM for these stronger robots. We show that the problem is still unsolvable if there are only two robots in the system. For 3 and 4 robots, we give algorithms that solve the GATHERING PROBLEM. For more than 4 robots, we present an algorithm that gathers the robots in finite time if they are not in a specific symmetric configuration at the beginning (biangular configuration). We show how to solve such initial configurations separately. However, the general solution of the GATHERING PROBLEM remains an open problem.
72 citations
21 Jul 2002
TL;DR: A black hole is considered: a highly harmful host that disposes of visiting agents upon their arrival, leaving no observable trace of such a destruction, and it is proved that with topological ignorance only two agents are needed and suffice, and the cost is Θ(n log n).
Abstract: Protecting agents from host attacks is a pressing security concern in networked environments supporting mobile agents. In this paper, we consider a black hole: a highly harmful host that disposes of visiting agents upon their arrival, leaving no observable trace of such a destruction. The task to identify the location of the harmful host is clearly dangerous for the searching agents. We study under what conditions and at what cost a team of autonomous asynchronous mobile agents can successfully accomplish this task; we are concerned with solutions that are generic (i.e., topology-independent). We study the size of the optimal solution (i.e., the minimum number of agents needed to locate the black hole), and the cost of the minimal solution (i.e., the number of moves performed by the agents executing a size-optimal solution protocol). We establish tight bounds on size and cost depending on the a priori knowledge the agents have about the network, and on the consistency of the local labellings. In particular, we prove that: with topological ignorance Δ + 1 agents are needed and suffice, and the cost is Θ(n2), where Δ is the maximal degree of a node and n is the number of the nodes in the network; with topological ignorance but in presence of sense of direction only two agents suffice and the cost is Θ(n2); and with complete topological knowledge only two agents suffice and the cost is Θ(n log n). All the upper-bound proofs are constructive.
65 citations
Proceedings Article•
01 Jan 2002
TL;DR: This paper presents a general strategy which allows two agents to locate the black hole with moves in hypercubes, cube-connected cycles, star graphs, wrapped butterflies, chordal rings, as well as in multidimensional meshes and tori of restricted diameter.
Abstract: Mobile agents operating in networked environments face threats from other agents as well as from the hosts (i.e., network sites) they visit. A black hole is a harmful host that destroys incoming agents without leaving any trace. To determine the location of such a harmful host is a dangerous but crucial task, called black hole search. The most important parameter for a solution strategy is the number of agents it requires (the size); the other parameter of interest is the total number of moves performed by the agents (the cost). Any solution requires moves in general networks; the same lower bound holds for rings. In this paper we show that this lower bound does not hold for hypercubes and related networks. In fact, we present a general strategy which allows two agents to locate the black hole with moves in hypercubes, cube-connected cycles, star graphs, wrapped butterflies, chordal rings, as well as in multidimensional meshes and tori of restricted diameter.
19 citations
01 Jan 2002
TL;DR: This paper presents a survey of some computational model for such simple robots, and discusses which problems can be successfully tackled at various levels of “stupidity” of the units.
Abstract: It is well known that sophisticated behavior can be exhibited by systems (or communities) composed of simple elements (members), each of which has only very limited intelligence and exhibits only simple behavior. Exploiting this emergent behavior in robotic systems is particularly important, since systems built according to this principle tend to cost less and be more robust and efficient than systems composed by more complex, powerful, intelligent – but less robust – units. This paper presents a survey of some computational model for such simple robots, and discusses which problems can be successfully tackled at various levels of “stupidity” of the units.
1 citations
DOI•
01 Jan 2002
TL;DR: The ability to detect how many robots are at a speciic point in the plane (multiplicity detection) is introduced and the general solution of the Gathering Problem remains an open problem.
Abstract: We study the problem of coordinating a set of autonomous mobile robots that can freely move in a two-dimensional plane; in particular, we want them to gather at a point not xed in advance (Gathering Problem). We introduce a model of weak robots (decentralized, asynchronous, no common knowledge, no identities, no central coordination, no direct communication, oblivious) which can observe the set of all points in the plane that are occupied by other robots. Based on this observation, a robot uses a deterministic algorithm to compute a destination, and moves there. The problem is unsolvable if the robots have no additional abilities. Therefore, we introduce the ability to detect how many robots are at a speciic point in the plane (multiplicity detection). For two robots, the problem remains unsolvable. For 3 and 4 robots, we give algorithms that solve the Gathering Problem. For more than 4 robots, we present an algorithm that gathers the robots if they are not in a speciic symmetric connguration at the beginning (totally symmetric connguration). We show how to solve such initial conngurations separately. However, the general solution of the Gathering Problem remains an open problem.
1 citations
15 Apr 2002
TL;DR: A novel method for parallel distribution sweeping on a clustered parallel machine with hierarchical local memories is presented, showing that it yields optimal computation, communication and memory access times for a number of geometry problems.
Abstract: This paper investigates the design of parallel algorithmic strategies that address the efficient use of both, memory hierarchies within each processor and a multilevel clustered structure of the interconnection between processors. In the past, these phenomena have usually been addressed separately. This paper is a first step towards parallel algorithmic strategies which address both at the same time. As a case study, we investigate the distribution sweeping method which has been very effective for the design of external memory algorithms for computational geometry problems. We present a novel method for parallel distribution sweeping on a clustered parallel machine with hierarchical local memories, showing that it yields optimal computation, communication and memory access times for a number of geometry problems.
1 citations