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arXiv · 1509.07716

The width of quadrangulations of the projective plane

Louis Esperet·Matěj Stehlík
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Abstract

We show that every $4$-chromatic graph on $n$ vertices, with no two vertex-disjoint odd cycles, has an odd cycle of length at most $\tfrac12\,(1+\sqrt{8n-7})$. Let $G$ be a non-bipartite quadrangulation of the projective plane on $n$ vertices. Our result immediately implies that $G$ has edge-width at most $\tfrac12\,(1+\sqrt{8n-7})$, which is sharp for infinitely many values of $n$. We also show that $G$ has face-width (equivalently, contains an odd cycle transversal of cardinality) at most $\tfrac14(1+\sqrt{16 n-15})$, which is a constant away from the optimal; we prove a lower bound of $\sqrt{n}$. Finally, we show that $G$ has an odd cycle transversal of size at most $\sqrt{2\Delta n}$ inducing a single edge, where $\Delta$ is the maximum degree. This last result partially answers a question of Nakamoto and Ozeki.

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Louis Esperet, Matěj Stehlík. 2015-09-25. The width of quadrangulations of the projective plane. https://doi.org/10.1002/jgt.22241

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