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Theoretical Computer Science
Volume 345, Issues 2-3, 22 November 2005, Pages 331-344
Mathematical Foundations of Computer Science 2004
 
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doi:10.1016/j.tcs.2005.07.014    How to Cite or Link Using DOI (Opens New Window)
Copyright © 2005 Elsevier B.V. All rights reserved.

Graph exploration by a finite automatonstar, open

Pierre Fraigniauda, 1, E-mail The Corresponding Author, David Ilcinkasa, 1, E-mail The Corresponding Author, Guy Peerb, E-mail The Corresponding Author, Andrzej Pelcc, 2, E-mail The Corresponding Author and David Pelegb, E-mail The Corresponding Author

aCNRS, LRI, Université Paris-Sud, France bDepartment of Computer Science, Weizmann Institute, Israel cDépt. of d’informatique, Univ. du Québec en Outaouais, Canada

Available online 15 August 2005.

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Abstract

A finite automaton, simply referred to as a robot, has to explore a graph whose nodes are unlabeled and whose edge ports are locally labeled at each node. The robot has no a priori knowledge of the topology of the graph or of its size. Its task is to traverse all the edges of the graph. We first show that, for any K-state robot and any dgreater-or-equal, slanted3, there exists a planar graph of maximum degree d with at most K+1 nodes that the robot cannot explore. This bound improves all previous bounds in the literature. More interestingly, we show that, in order to explore all graphs of diameter D and maximum degree d, a robot needs Ω(Dlogd) memory bits, even if we restrict the exploration to planar graphs. This latter bound is tight. Indeed, a simple DFS up to depth D+1 enables a robot to explore any graph of diameter D and maximum degree d using a memory of size O(Dlogd) bits. We thus prove that the worst case space complexity of graph exploration is Θ(Dlogd) bits.

Keywords: Graph exploration; Labyrinth; Finite automaton; Mobile agent; Robot


Theoretical Computer Science
Volume 345, Issues 2-3, 22 November 2005, Pages 331-344
Mathematical Foundations of Computer Science 2004
 
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