Internally-disjoint Directed Pendant Steiner Trees in Digraphs

Abstract: For a digraph $D=(V(D),A(D))$ and a set $S\subseteq V(D)$ with $|S|\geq 2$ and $r\in S$, a directed pendant $(S,r)$-Steiner tree (or, simply, a pendant $(S,r)$-tree) is an out-tree $T$ rooted at $r$ such that $S\subseteq V(T)$ and each vertex of $S$ has degree one in $T$. Two pendant $(S,r)$-trees are called internally-disjoint if they are arc-disjoint and their common vertex set is exactly $S$. The goal of the {\sc Internally-disjoint Directed Pendant Steiner Tree Packing (IDPSTP)} problem is to find a largest collection of pairwise internally-disjoint pendant $(S,r)$-trees in $D$. We use $\tau_{S,r}(D)$ to denote the maximum number of pairwise internally-disjoint pendant $(S,r)$-trees in $D$ and define the directed pendant-tree $k$-connectivity of $D$ as \begin{align*} \tau_{k}(D)=\min{\tau_{S,r}(D)\mid S\subseteq V(D),|S|=k,r\in S}. \end{align*} IDPSTP is a restriction of the {\sc Internally-disjoint Directed Steiner Tree Packing} problem studied by Cheriyan and Salavatipour [Algorithmica, 2006] and Sun and Yeo [JGT, 2023]. The directed pendant-tree $k$-connectivity extends the concept of pendant-tree $k$-connectivity in undirected graphs studied by Hager [JCTB, 1985] and could be seen as a generalization of classical vertex-connectivity of digraphs. In this paper, we completely determine the computational complexity for the parameter $\tau_{S,r}(D)$ on Eulerian digraphs and symmetric digraphs. We also give sharp bounds and values for the parameter $\tau_{k}(D)$.