Distributed Computing by Mobile Robots: Uniform Circle Formation

TL;DR

This paper proves that a set of simple, asynchronous, and memoryless mobile robots can deterministically arrange themselves into a regular circle from any initial configuration, resolving a long-standing open problem.

Contribution

It demonstrates that uniform circle formation is possible for any initial configuration of simple robots, removing previous restrictions and assumptions.

Findings

Uniform circle formation is solvable from any initial configuration.

Asynchrony does not hinder pattern formation capabilities.

Additional powers like chirality and rigidity are unnecessary for the task.

Abstract

Consider a set of $n$ 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.