| Popis: |
Pursuit-evasion games study the number of cops needed to capture the robber in a game played on a graph, in which the cops and the robber move alternatively to neighbouring vertices, and the robber is captured if a cop steps on the vertex the robber is in. A common tool in analyzing this cop number of a graph is a cop moving along a shortest path in a graph, thus preventing the robber to step onto this path. We generalize this approach by introducing a shadow of the robber, the maximal set of vertices from which the cop parries the protected subgraph. In this context, the robber becomes an intruder and the cop becomes the guard. We show that the shadow can be computed in polynomial time, implying polynomial time algorithms for computing both a successful guard as well as a successful intruder, whichever exists. Furthermore, we show that shadow function generalizes the concept of graph retractions. In some cases, this implies a polynomially computable certification of the negative answer to the NP-complete problem of existence of a retraction to a given subgraph. |
