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arXiv · 1603.04266

Cops and Robbers ordinals of cop-win trees

Anthony Bonato·Przemysław Gordinowicz·Gena Hahn
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Abstract

A relational characterization of cop-win graphs was provided by Nowakowski and Winkler in their seminal paper on the game of Cops and Robbers. As a by-product of that characterization, each cop-win graph is assigned a unique ordinal, which we refer to as a CR-ordinal. For finite graphs, CR-ordinals correspond to the length of the game assuming optimal play, with the cop beginning the game in a least favourable initial position. For infinite graphs, however, the possible values of CR-ordinals have not been considered in the literature until the present work. We classify the CR-ordinals of cop-win trees as either a finite ordinal, or those of the form $\alpha + \omega$, where $\alpha$ is a limit ordinal. For general infinite cop-win graphs, we provide an example whose CR-ordinal is not of this form. We finish with some problems on characterizing the CR-ordinals in the general case of cop-win graphs.

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Anthony Bonato, Przemysław Gordinowicz, Gena Hahn. 2016-03-14. Cops and Robbers ordinals of cop-win trees. https://doi.org/10.1016/j.disc.2016.12.019

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