Collective Tree Exploration via Potential Function Method

We study the problem of collective tree exploration (CTE) where a team of $k$ agents is tasked to traverse all the edges of an unknown tree as fast as possible, assuming complete communication between the agents. In this paper, we present an algorithm performing collective tree exploration in only $2n/k+O(kD)$ rounds, where $n$ is the number of nodes in the tree, and $D$ is the tree depth. This leads to a competitive ratio of $O(\sqrt{k})$ for collective tree exploration, the first polynomial improvement over the initial $O(k/\log(k))$ ratio of [FGKP06]. Our analysis relies on a game with robots at the leaves of a continuously growing tree, which is presented in a similar manner as the `evolving tree game' of [BCR22], though its analysis and applications differ significantly. This game extends the `tree-mining game' (TM) of [Cos23] and leads to guarantees for an asynchronous extension of collective tree exploration (ACTE). Another surprising consequence of our results is the existence of algorithms $\{A_k\}_{k\in \mathbb{N}}$ for layered tree traversal (LTT) with cost at most $2L/k+O(kD)$, where $L$ is the sum of edge lengths and $D$ is the tree depth. For the case of layered trees of width $w$ and unit edge lengths, our guarantee is thus in $O(\sqrt{w}D)$.