Turan problems for $k$-geodetic digraphs

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 $\le k$ 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 Turán problems for the number of directed cycles and paths in $k$-geodetic digraphs.