Perfect Out-forests and Steiner Cycle Packing in Digraphs

Abstract: In this paper, we study the complexity of two types of digraph packing problems: perfect out-forests problem and Steiner cycle packing problem. For the perfect out-forest problem, we prove that it is NP-hard to decide whether a given strong digraph contains a 1-perfect out-forest. However, when restricted to a semicomplete digraph $D$, the problem of deciding whether $D$ contains an $i$-perfect out-forest becomes polynomial-time solvable, where $i\in {0,1}$. We also prove that it is NP-hard to find a 0-perfect out-forest of maximum size in a connected acyclic digraph, and it is NP-hard to find a 1-perfect out-forest of maximum size in a connected digraph. For the Steiner cycle packing problem, when both $k\geq 2, \ell\geq 1$ are fixed integers, we show that the problem of deciding whether there are at least $\ell$ internally disjoint directed $S$-Steiner cycles in an Eulerian digraph $D$ is NP-complete, where $S\subseteq V(D)$ and $|S|=k$. However, when we consider the class of symmetric digraphs, the problem becomes polynomial-time solvable. We also show that the problem of deciding whether there are at least $\ell$ arc-disjoint directed $S$-Steiner cycles in a given digraph $D$ is NP-complete, where $S\subseteq V(D)$ and $|S|=k$.