On networks with order close to the Moore bound

TL;DR

This paper investigates bounds on the size of mixed graphs with specific diameter or geodetic girth, introduces new examples of such graphs, and proves a conjecture about the structure of nearly optimal $k$-geodetic mixed graphs.

Contribution

It provides new bounds on the order of mixed graphs with given parameters, constructs new geodetic cages, and proves a conjecture on the structure of $k$-geodetic mixed graphs with excess one.

Findings

Any $k$-geodetic mixed graph with excess one has geodetic girth two.

Such graphs must be totally regular.

New bounds and examples of mixed geodetic cages are presented.

Abstract

The degree/diameter problem for mixed graphs asks for the largest possible order of a mixed graph with given diameter and degree parameters. Similarly the \emph{degree/geodecity} problem concerns the smallest order of a $k$-geodetic mixed graph with given minimum undirected and directed degrees; this is a generalisation of the classical degree/girth problem. In this paper we present new bounds on the order of mixed graphs with given diameter or geodetic girth and exhibit new examples of directed and mixed geodetic cages. In particular, we show that any $k$-geodetic mixed graph with excess one must have geodetic girth two and be totally regular, thereby proving an earlier conjecture of the authors.