On the girth cycles of the bipartite graph $D(k,q)$

TL;DR

This paper determines the exact girth cycles of the algebraic bipartite graph D(k,q) for specific small values of k and q, confirming conjectures and extending previous results in graph theory.

Contribution

It explicitly characterizes all girth cycles of D(k,q) for certain small k and q, advancing understanding of these graphs' structure.

Findings

Girth cycles for 3 ≤ k ≤ 5, q > 3 are fully characterized.

Girth cycles for 3 ≤ k ≤ 8, q=3 are determined.

Confirmed and extended conjectures on girth of D(k,q).

Abstract

For integer $k\geq2$ and prime power $q$, the algebraic bipartite graph $D(k,q)$ proposed by Lazebnik and Ustimenko (1995) is meaningful not only in extremal graph theory but also in coding theory and cryptography. This graph is $q$-regular, edge-transitive and of girth at least $k+4$. For its exact girth $g=g(D(k,q))$, F\"{u}redi et al. (1995) conjectured $g=k+5$ for odd $k$ and $q\geq4$. This conjecture was shown to be valid in 2016 when $(k+5)/2$ is the product of an arbitrary factor of $q-1$ and an arbitrary power of the characteristic of $\mathbb{F}_q$. In this paper, we determine all the girth cycles of $D(k,q)$ for $3\leq k\leq 5$, $q>3$, and those for $3\leq k\leq8$, $q=3$.