An acyclic digraph is a directed graph containing no directed cycles, also known as a
directed acyclic graph or a "
DAG." Every acyclic digraph has at least one node of outdegree 0. The numbers of acyclic digraphs on n = 1, 2, … vertices are 1, 2, 6, 31, 302, 5984, … (
Sloane's A003087).