Some results on similar configurations in subsets of $\mathbb{F}_q^d$

Abstract

In this paper, we study problems about the similar configurations in $\mathbb{F}_q^d$. Let $G=(V, E)$ be a graph, where $V=\{1, 2, \ldots, n\}$ and $E\subseteq{V\choose2}$. For a set $\mathcal{E}$ in $\mathbb{F}_q^d$, we say that $\mathcal{E}$ contains a pair of $G$ with dilation ratio $r$ if there exist distinct $\boldsymbol{x}_1, \boldsymbol{x}_2, \ldots, \boldsymbol{x}_n\in\mathcal{E}$ and distinct $\boldsymbol{y}_1, \boldsymbol{y}_2, \ldots, \boldsymbol{y}_n\in\mathcal{E}$ such that $\|\boldsymbol{y}_i-\boldsymbol{y}_{j}\|=r\|\boldsymbol{x}_i-\boldsymbol{x}_j\|\neq0$ whenever $\{i, j\}\in E$, where $\|\boldsymbol{x}\|:=x_1^2+x_2^2+\cdots+x_d^2$ for $\boldsymbol{x}=(x_1, x_2, \ldots, x_d)\in\mathbb{F}_q^d$. We show that if $\mathcal{E}$ has size at least $C_kq^{d/2}$, then $\mathcal{E}$ contains a pair of $k$-stars with dilation ratio $r$, and that if $\mathcal{E}$ has size at least $C\cdot\min\left\{q^{(2d+1)/3}, \max\left\{q^3, q^{d/2}\right\}\right\}$, then $\mathcal{E}$ contains a pair of $4$-paths with dilation ratio $r$. Our method is based on enumerative combinatorics and graph theory.