A Note on the Cops and Robber Game on Graphs Embedded in Non-Orientable Surfaces

Abstract

We consider the two-player, complete information game of Cops and Robber played on undirected, finite, reflexive graphs. A number of cops and one robber are positioned on vertices and take turns in sliding along edges. The cops win if, after a move, a cop and the robber are on the same vertex. The minimum number of cops needed to catch the robber on a graph is called the cop number of that graph. Let c(g) be the supremum over all cop numbers of graphs embeddable in a closed orientable surface of genus g, and likewise c̃(g) for non-orientable surfaces. It is known (Andreae, 1986) that, for a fixed surface, the maximum over all cop numbers of graphs embeddable in this surface is finite. More precisely, Quilliot (1985) showed that c(g) ≤ 2g + 3, and Schröder (2001) sharpened this to c(g)≤ 3/2g + 3. In his paper, Andreae gave the bound c̃(g) ∈ O(g) with a weak constant, and posed the question whether a stronger bound can be obtained. Nowakowski & Schröder (1997) obtained c̃(g) ≤ 2g+1. In this short note, we show c̃(g) ≤ c(g-1), for any g ≥ 1. As a corollary, using Schröder's results, we obtain the following: the maximum cop number of graphs embeddable in the projective plane is 3, the maximum cop number of graphs embeddable in the Klein Bottle is at most 4, c̃(3) ≤ 5, and tilde c̃(g) ≤ 3/2g + 3/2 for all other g. © 2012 Springer Japan.