Cops and Robbers on Dynamic Graphs: Offline and Online Case

• Published in: Discrete mathematics and theoretical computer science Vol. 25:1; no. Discrete Algorithms; pp. 1 - 20
• Main Authors: Balev, Stefan, Jiménez Laredo, Juan, Lamprou, Ioannis, Pigné, Yoann, Sanlaville, Eric
• Format: Journal Article
• Language: English
• Published: Nancy DMTCS 01.01.2023; Discrete Mathematics & Theoretical Computer Science
• Subjects: [info.info-dm]computer science [cs]/discrete mathematics [cs.dm]; [info.info-ro]computer science [cs]/operations research [cs.ro]; Algorithms; Apexes; Computer Science; cops and robbers; Discrete Mathematics; dynamic graphs; Evolution; Games; Graph theory; Graphs; offline; online; Operations Research; Robbery; sparse; cops and robbers; dynamic graphs; offline; online; sparse
• ISSN: 1365-8050; 1462-7264; 1365-8050
• DOI: 10.46298/dmtcs.8784

We examine the classic game of Cops and Robbers played on models of dynamic graphs, that is, graphs evolving over discrete time steps. At each time step, a graph instance is generated as a subgraph of the underlying graph of the model. The cops and the robber take their turns on the current graph instance. The cops win if they can capture the robber at some point in time. Otherwise, the robber wins. In the offline case, the players are fully aware of the evolution sequence, up to some finite time horizon T. We provide a O(n 2k+1 T) algorithm to decide whether a given evolution sequence for an underlying graph with n vertices is k-cop-win via a reduction to a reachability game. In the online case, there is no knowledge of the evolution sequence, and the game might go on forever. Also, each generated instance is required to be connected. We provide a nearly tight characterization for sparse underlying graphs, i.e., with at most linear number of edges. We prove λ + 1 cops suffice to capture the robber in any underlying graph with n − 1 + λ edges. Further, we define a family of underlying graphs with n−1+λ edges where λ−1 cops are necessary (and sufficient) for capture.