The Meeting problem for @math searchers in a polygon @math (possibly with holes) consists in making the searchers move within @math , according to a distributed algorithm, in such a way that at least two of them eventually come to see each other, regardless of their initial positions. The polygon is initially unknown to the searchers, and its edges obstruct both movement and vision. Depending on the shape of @math , we minimize the number of searchers @math for which the Meeting problem is solvable. Specifically, if @math has a rotational symmetry of order @math (where @math corresponds to no rotational symmetry), we prove that @math searchers are sufficient, and the bound is tight. Furthermore, we give an improved algorithm that optimally solves the Meeting problem with @math searchers in all polygons whose barycenter is not in a hole (which includes the polygons with no holes). Our algorithms can be implemented in a variety of standard models of mobile robots operating in Look-Compute-Move cycles. For instance, if the searchers have memory but are anonymous, asynchronous, and have no agreement on a coordinate system or a notion of clockwise direction, then our algorithms work even if the initial memory contents of the searchers are arbitrary and possibly misleading. Moreover, oblivious searchers can execute our algorithms as well, encoding information by carefully positioning themselves within the polygon. This code is computable with basic arithmetic operations, and each searcher can geometrically construct its own destination point at each cycle using only a compass. We stress that such memoryless searchers may be located anywhere in the polygon when the execution begins, and hence the information they initially encode is arbitrary. Our algorithms use a self-stabilizing map construction subroutine which is of independent interest.
@cite_29: We study the impact that persistent memory has on the classical rendezvous problem of two mobile computational entities, called robots, in the plane. It is well known that, without additional assumptions, rendezvous is impossible if the entities are oblivious (i.e., have no persistent memory) even if the system is semi-synchronous (SSynch). It has been recently shown that rendezvous is possible even if the system is asynchronous (ASynch) if each robot is endowed with O ( 1 ) bits of persistent memory, can transmit O ( 1 ) bits in each cycle, and can remember (i.e., can persistently store) the last received transmission. This setting is overly powerful.In this paper we weaken that setting in two different ways: (1) by maintaining the O ( 1 ) bits of persistent memory but removing the communication capabilities, a setting we call finite-state (FState); and (2) by maintaining the ability of transmitting O ( 1 ) bits and remembering the last received message, but removing the ability of an agent to remember its previous activities, a setting we call finite-communication (FComm). Note that, even though its use is very different, in both settings, the amount of persistent memory of a robot is a constant number of bits.We investigate the rendezvous problem in these two weaker settings. We model both settings as a system of robots endowed with visible lights, each with a constant number of colors: in FState, a robot can only see its own light, while in FComm a robot can only see the other robot's light. Among other things, we prove that, with rigid movements, finite-state robots can rendezvous in SSynch, and that finite-communication robots are able to rendezvous even in ASynch. All proofs are constructive: in each setting, we present a protocol that allows the two robots to rendezvous in finite time.
@cite_7: 
@cite_8: In this paper, we investigate the possibility to deterministically solve the gathering problem starting from an arbitrary configuration with weak robots, i.e., anonymous, autonomous, disoriented, oblivious, and devoid of means of communication. By starting from an arbitrary configuration, we mean that robots are not required to be located at distinct positions in the initial configuration. We introduce strong multiplicity detection as the ability for the robots to detect the exact number of robots located at a given position. We show that with strong multiplicity detection, there exists a deterministic algorithm solving the gathering problem starting from an arbitrary configuration for n robots if, and only if, n is odd.
Fault-tolerant Gathering has been investigated in @cite_7 @cite_8 for oblivious robots with unlimited visibility. Rendezvous has been investigated also when the robots have a constant amount of visible memory'' @cite_29 .