Cycles of Arbitrary Length in Distance Graphs on $\mathbb F_q^d$

A. Iosevich ; G. Jardine ; B. McDonald

Informatics and Automation, Analytic and Combinatorial Number Theory, Tome 314 (2021), pp. 31-48

Résumé

For $E \subset \mathbb F_q^d$, $d \ge 2$, where $\mathbb F_q$ is the finite field with $q$ elements, we consider the distance graph $\mathcal G^{\text {dist}}_t(E)$, $t\neq 0$, where the vertices are the elements of $E$, and two vertices $x$, $y$ are connected by an edge if $\|x-y\| \equiv (x_1-y_1)^2+\dots +(x_d-y_d)^2=t$. We prove that if $|E| \ge C_k q^{\frac {d+2}{2}}$, then $\mathcal G^{\text {dist}}_t(E)$ contains a statistically correct number of cycles of length $k$. We are also going to consider the dot-product graph $\mathcal G^{\text {prod}}_t(E)$, $t\neq 0$, where the vertices are the elements of $E$, and two vertices $x$, $y$ are connected by an edge if $x\cdot y \equiv x_1y_1+\dots +x_dy_d=t$. We obtain similar results in this case using more sophisticated methods necessitated by the fact that the function $x\cdot y$ is not translation invariant. The exponent $\frac {d+2}{2}$ is improved for sufficiently long cycles.