Triangle-free induced subgraphs of polarity graphs

Abstract

Given a finite projective plane $\Pi$ and a polarity $\theta$ of $\Pi$, the corresponding polarity graph is the graph whose vertices are the points of $\Pi$. Two distinct vertices $p$ and $p^{\prime }$ are adjacent if $p$ is incident to $\theta (p^{\prime })$. Polarity graphs have been used in a variety of extremal problems, perhaps the most well-known being the Turán number of the cycle of length four. We investigate the problem of finding the maximum number of vertices in an induced triangle-free subgraph of a polarity graph. Mubayi and Williford showed that when $\Pi$ is the projective geometry $PG(2,q)$ and $\theta$ is the orthogonal polarity, an induced triangle-free subgraph has at most $\frac{1}{2}{q}^2+O(q^{3/2})$ vertices. We generalize this result to all polarity graphs, and provide some interesting computational results that are relevant to an unresolved conjecture of Mubayi and Williford.

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