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arXiv · 2501.01697

Sets preserved by a large subgroup of the special linear group

Le Quang Hung·Thang Pham·Kaloyan Slavov
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Abstract

Let $E$ be a subset of the affine plane over a finite field $\mathbb{F}_q$. We bound the size of the subgroup of $SL_2(\mathbb{F}_q)$ that preserves $E$. As a consequence, we show that if $E$ has size $\ll q^\alpha$ and is preserved by $\gg q^\beta$ elements of $SL_2(\mathbb{F}_q)$ with $\beta\geq 3\alpha/2$, then $E$ is contained in a line. This result is sharp in general, and will be proved by using combinatorial arguments and applying a point-line incidence bound in $\mathbb{F}_q^3$ due to Mockenhaupt and Tao (2004).

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Le Quang Hung, Thang Pham, Kaloyan Slavov. 2025-01-03. Sets preserved by a large subgroup of the special linear group. https://arxiv.org/abs/2501.01697

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