Given a digraph $G(V,A)$ and a number $k$, we want to find two vertex subsets $S,T\subseteq V$ such that:
- For every $v\notin S\cup T$, $v$ has no arcs coming to $S$, and no arcs coming from $T$. In other words, from $v$'s point of view, $S$ is the source, $T$ is the terminal. Hence their names.
So, can this be solved by an efficient algorithm or it is NP-complete?