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arXiv · 2502.11421

On rigid regular graphs and a problem of Babai and Pultr

Kolja Knauer·Gil Puig i Surroca
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Abstract

A graph is \textit{rigid} if it only admits the identity endomorphism. We show that for every $d\ge 3$ there exist infinitely many mutually rigid $d$-regular graphs of arbitrary odd girth $g\geq 7$. Moreover, we determine the minimum order of a rigid $d$-regular graph for every $d\ge 3$. This provides strong positive answers to a question of van der Zypen [https://mathoverflow.net/q/296483, https://mathoverflow.net/q/321108]. Further, we use our construction to show that every finite monoid is isomorphic to the endomorphism monoid of a regular graph. This solves a problem of Babai and Pultr [J. Comb.~Theory, Ser.~B, 1980].

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Kolja Knauer, Gil Puig i Surroca. 2025-02-17. On rigid regular graphs and a problem of Babai and Pultr. https://arxiv.org/abs/2502.11421

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