# Convex node set

In a digraph D=(V,A) with disjoint node sets X and Y, we say that Y is reachable from X if there is a directed path in D whose first node is in X and last node is in Y. A subset $T\subseteq V$ is called convex if there is no node v in V-T so that T is reachable from {v} and {v} is reachable from T.