English

The 3x3 rooks graph is the unique smallest graph with lazy cop number 3

Combinatorics 2016-06-29 v1 Discrete Mathematics

Abstract

In the ordinary version of the pursuit-evasion game "cops and robbers", a team of cops and a robber occupy vertices of a graph and alternately move along the graph's edges, with perfect information about each other. If a cop lands on the robber, the cops win; if the robber can evade the cops indefinitely, he wins. In the variant "lazy cops and robbers", the cops may only choose one member of their squad to make a move when it's their turn. The minimum number of cops (respectively lazy cops) required to catch the robber is called the "cop number" (resp. "lazy cop number") of G and is denoted c(G)c(G) (resp. cL(G)c_L(G)). Previous work by Beveridge at al. has shown that the Petersen graph is the unique graph on ten vertices with c(G)=3c(G)=3, and all graphs on nine or fewer vertices have c(G)2c(G)\leq 2. (This was a self-contained mathematical proof of a result found by computational search by Baird and Bonato.) In this article, we prove a similar result for lazy cops, namely that the 3x3 rooks graph (K3K3K_3\square K_3) is the unique graph on nine vertices which requires three lazy cops, and a graph on eight or fewer vertices requires at most two lazy cops.

Keywords

Cite

@article{arxiv.1606.08485,
  title  = {The 3x3 rooks graph is the unique smallest graph with lazy cop number 3},
  author = {Brendan W. Sullivan and Nikolas Townsend and Mikayla Werzanski},
  journal= {arXiv preprint arXiv:1606.08485},
  year   = {2016}
}

Comments

24 pages, 14 figures