A127909 Number of different digraphs on n unlabeled nodes which are not graphs.

0, 0, 1, 12, 207, 9574, 1540788, 882032396, 1793359180502, 13027956824124884, 341260431952960575184, 32522909385055885092199576, 11366745430825400574268802831632, 14669085692712929869037045573284852976, 70315656615234999521385506526925748433982432

Offset

0,4

Comments

A digraph is (isomorphic to) a graph if every pair of points a, b joined by a directed edge (a,b) also has the reverse directed edge (b,a). A digraph which is not a graph is a digraph with at least one pair of points which have only one directed edge connecting them.

Links

Table of n, a(n) for n=0..14.

Formula

a(n) = A000273(n) - A000088(n).

Example

a(2) = 1 because with two points a and b, either there are no edges connecting them, or there is one directed edge between them, or there is a bidirectional pair of edges between them; only the case with one directed edge is the unique 2-point digraph which is not a graph.
a(3) = 12 because A000273(3) = number of directed graphs (or digraphs) with 3 nodes = 16; A000088(3) = number of graphs on 3 unlabeled nodes = 4; 16-4 = 12.

Crossrefs

Cf. A000088, A000273, A127910-A127915.

Sequence in context: A297311 A217703 A245911 * A307691 A129466 A259516

Adjacent sequences: A127906 A127907 A127908 * A127910 A127911 A127912

Keyword

easy,nonn

Author

Jonathan Vos Post, Feb 06 2007

Status

approved