# Digraphs with degree two and excess two are diregular

## TL;DR

This paper proves that digraphs with degree two and excess two are necessarily diregular for diameters at least two, extending the understanding of the structure of near-Moore digraphs.

## Contribution

It establishes that digraphs with minimum out-degree two and excess two are diregular for all diameters at least two, a new result in the study of digraph regularity.

## Key findings

- Digraphs with degree two and excess two are diregular for k ≥ 2.
- Extends the class of known diregular digraphs with small excess.
- Provides structural insights into near-Moore digraphs.

## Abstract

A $k$-geodetic digraph with minimum out-degree $d$ has excess $\epsilon $ if it has order $M(d,k) + \epsilon $, where $M(d,k)$ represents the Moore bound for out-degree $d$ and diameter $k$. For given $\epsilon $, it is simple to show that any such digraph must be out-regular with degree $d$ for sufficiently large $d$ and $k$. However, proving in-regularity is in general non-trivial. It has recently been shown that any digraph with excess $\epsilon = 1$ must be diregular. In this paper we prove that digraphs with minimum out-degree $d = 2$ and excess $\epsilon = 2$ are diregular for $k \geq 2$.