Distributed computing by mobile robots: uniform circle formation
Consider a set of n finite set of simple autonomous mobile robots (asynchronous, no common coordinate system, no identities, no central coordination, no direct communication, no memory of the past, non-rigid, deterministic) initially in distinct locations, moving freely in the plane and able to sense the positions of the other robots. We study the primitive task of the robots arranging themselves on the vertices of a regular n-gon not fixed in advance (Uniform Circle Formation). In the literature, the existing algorithmic contributions are limited to conveniently restricted sets of initial configurations of the robots and to more powerful robots. The question of whether such simple robots could deterministically form a uniform circle has remained open. In this paper, we constructively prove that indeed the Uniform Circle Formation problem is solvable for any initial configuration in which the robots are in distinct locations, without any additional assumption (if two robots are in the same location, the problem is easily seen to be unsolvable). In addition to closing a long-standing problem, the result of this paper also implies that, for pattern formation, asynchrony is not a computational handicap, and that additional powers such as chirality and rigidity are computationally irrelevant.
KeywordsAutonomous mobile robots Uniform circle formation
The authors would like to thank the anonymous reviewers for helping improve the readability of the paper, Marc-André Paris-Cloutier for many helpful discussions and insights, and Peter Widmayer and Vincenzo Gervasi for sharing some of the fun and frustrations emerging from investigating this problem. This work has been supported in part by the Natural Sciences and Engineering Research Council of Canada under the Discovery Grants program, by Professor Flocchini’s University Research Chair, and by project PRA_2016_64 Through the fog funded by the University of Pisa.
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