Turan problems for $k$-geodetic digraphs

TL;DR

This paper investigates the maximum size of $k$-geodetic digraphs with given order, providing solutions for strongly-connected cases when $k=2$, and explores Turán-type problems related to cycles and paths.

Contribution

It determines the maximum size of $k$-geodetic digraphs, solves the strongly-connected case for $k=2$, and proposes a conjecture for larger $k$ with related Turán problems.

Findings

Maximum size of $k$-geodetic digraphs established

Solution for strongly-connected case when $k=2$

Conjecture for extremal structures for larger $k$

Abstract

A digraph $G$ is \emph{$k$-geodetic} if for any pair of (not necessarily distinct) vertices $u,v \in V(G)$ there is at most one walk of length $\leq k$ from $u$ to $v$ in $G$. In this paper we determine the largest possible size of a $k$-geodetic digraph with given order. We then consider the more difficult problem of the largest size of a strongly-connected $k$-geodetic digraph with given order, solving this problem for $k = 2$ and giving a construction which we conjecture to be extremal for larger $k$. We close with some results on generalised Tur\'{a}n problems for the number of directed cycles and paths in $k$-geodetic digraphs.