The Game of Cop and Robbers

In the game of Cops and Robbers , there is a map consisting of nodes and edges.

  • The cop and the robber are each on some node to begin with.
  • The cop and the robber take turns to make moves.
  • In each turn, the cop or the robber can either stay where he is, or go to a node that is connected by an edge.

The cop wins if he can manage to capture the robber , i.e. be in the same vertex as the robber.

Here are a few maps on which Cops and Robbers can be played:

On one of these maps, it is never possible for the cop to win if the robber plays well. Which one is it?

A B C

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3 solutions

Michael Mendrin
Apr 14, 2017

In both maps B and C, the cop can be in a node where he can catch the crook no matter what move the crook makes. Only in map A the cop can't do that.

I'm confused about this problem, if the cop and robber both start on the same node shouldn't the cop win instantly?

Alex Li - 3 years, 11 months ago
Annie Li
Apr 19, 2017

In the diagram A, the robber can just go round and round either ways.

Odinrawo201 Rom
Apr 28, 2017

In map A, the robber can always be opposite from the cop, and there would never be a way to catch him. However, in map B all of the nodes are connected, so the cop can reach the robber wherever he is(at least after one move). In map c, the only way the robber could avoid the cop would be to trap himself in a corner were he can't go out if the cop is opportunistic.

Yep. How do we identify graphs in which the cop always wins?

Agnishom Chattopadhyay - 4 years, 1 month ago

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