Fast Neighborhood Rendezvous

In the rendezvous problem, two computing entities (called agents) located at different vertices in a graph have to meet at the same vertex. In this paper, we consider the synchronous neighborhood rendezvous problem, where the agents are initially located at two adjacent vertices. While this problem can be trivially solved in O(Δ) rounds (Δ is the maximum degree of the graph), it is highly challenging to reveal whether that problem can be solved in o(Δ) rounds, even assuming the rich computational capability of agents. The only known result is that the time complexity of O(√n) rounds is achievable if the graph is complete and agents are probabilistic, asymmetric, and can use whiteboards placed at vertices. Our main contribution is to clarify the situation (with respect to computational models and graph classes) admitting such a sublinear-time rendezvous algorithm. More precisely, we present two algorithms achieving fast rendezvous additionally assuming bounded minimum degree, unique vertex identifier, and accessibility to neighborhood IDs. The first algorithm runs within Õ(√(nΔ/δ) + n/δ) rounds for graphs of the minimum degree larger than √n, where n is the number of vertices in the graph, and δ is the minimum degree of the graph. The second algorithm assumes that the largest vertex ID is O(n), and achieves Õ(n/√δ)-round time complexity without using whiteboards. These algorithms attain o(Δ)-round complexity in the case of δ = ω(√n log n) and δ = ω(n
^2/3
log
^4/3
n) respectively. We also prove that three unconventional assumptions of our algorithm, bounded minimum degree, accessibility to neighborhood IDs, and initial distance one, are all inherently necessary for attaining fast rendezvous. That is, one can obtain the Ω(n)-round lower bound if either one of them is removed.