Consider a set of k identical asynchronous mobile agents located in an anonymous ring of n nodes. The classical Gather (or Rendezvous) problem requires all agents to meet at the same node, not a priori decided, within a finite amount of time. This problem has been studied assuming that the network is safe for the agents. In this paper, we consider the presence in the ring of a stationary process located at a node that disables any incoming agent without leaving any trace. Such a dangerous node is known in the literature as a black hole, and the determination of its location has been extensively investigated. The presence of the black hole makes it deterministically unfeasible for all agents to gather. So, the research concern is to determine how many agents can gather and under what conditions. In this paper we establish a complete characterization of the conditions under which the problem can be solved. In particular, we determine the maximum number of agents that can be guaranteed to gather in the same location depending on whether k or n is unknown (at least one must be known). These results are tight: in each case, gathering with one more agent is deterministically unfeasible. All our possibility proofs are constructive: we provide mobile agent algorithms that allow the agents to gather within a predefined distance under the specified conditions. The analysis of the time costs of these algorithms show that they are optimal. Our gathering algorithm for the case of unknown k is also a solution for the black hole location problem. Interestingly, its bounded time complexity is (Formula presented.) ; this is a significant improvement over the existing (Formula presented.) bounded time complexity.
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