The number of ordered pairs (G,S) where G is a simple labeled graph of order n and S is a subset of the vertices of G such that every element (vertex) in S is in the same connected component of G.

Every graph paired with the empty set is included in this count. Every graph paired with a single vertex is also included.

a(n)/(2^binomial(n,2)*2^n) is the probability that a random simple labeled graph contains a random subset of its vertices in a single connected component.