The Markov Factorization Theorem.

A graph consists of a set V of vertices (or nodes) and a set E of edges (or links) that connect some pairs of vertices... A directed graph is a graph consisting of directed edges; i.e. each edge is marked by a single arrowhead. A directed path in a graph is a sequence edges, each edge pointing to a node from which the next edge emerges. A path in a graph is a sequence (directed or not) of edges such that each pair of consecutive edges in the sequence share one node. A cycle is any directed path that starts and ends at the same node. A graph that contains no directed cycles is called acycli