Connectivity keeping paths in digraphs

Abstract

Mader conjectured that every $k$-strong digraph $D$ with minimum semidegree $δ^0(D)\ge 2k+m-1$ contains a dipath $P$ of order $m$ such that $D-V(P)$ remains $k$-strong. For $k=1$, he obtained the weaker bound $δ^0(D)\ge 2m$. We confirm the conjecture for $k=1$ by showing that the sharp bound $δ^0(D)\ge m+1$ suffices. As a consequence, we show that for every integer $m\ge2$, every strongly connected digraph $D$ with $δ^0(D)\ge\max\{2,m-1\}$ contains a dipath $P$ of order $m$ such that $D-A(P)$ is strongly connected.