Online Treasure Hunt in Vertex-Permuted Dynamic Rings

Abstract: We study the problem of treasure hunt by a group of $k \geq 1$ agents in vertex-permuted dynamic rings (VP). In this model, the $n$ vertices remain on a ring but are permuted at each time step. We first show that treasure hunt is impossible for any $k \leq n-3$ agents, if there are no restrictions on the sequence of permutations used in the dynamic ring. We then study the $VP(δ)$ setting, in which for every pair $i, j$ of vertices, the edge $(i, j)$ is guaranteed to appear within $δ$ steps. We show that the class $VP(δ)$ is feasible only for $δ\geq \left\lceil \frac{n-1}{2}\right\rceil$. For the one-agent case, we show a tight bound of $Θ(δn)$ on the worst-case search time as well as competitive ratio of any online algorithm for treasure hunt, provided $δ\geq 2n$. We then give an optimal algorithm for $k$ agents, thereby showing that $k$ agents can obtain a speedup of $k$ on the worst-case search time. Finally, in the R-VP setting, in which in every step, the vertices are arranged as a ring according to a random permutation, we show that treasure hunt takes expected $Θ(n)$ steps against an oblivious adversary and $Θ(n \log n)$ steps against an adaptive adversary.