

A306892


Isomorphism classes of connected 2regular digraphs on n nodes, allowing multiarcs and loops.


3




OFFSET

0,3


COMMENTS

The graphs are directed, connected and have indegree=outdegree=2 at each node. Multiarcs (connecting two nodes with the same sense of heading) and loops (edges
connecting a node to itself) are permitted.
The sequence of the same family of graphs which are not necessarily connected is 1, 1, 3, 8, 25, 85, ... (obtained by a Euler Transformation of this sequence) A006372.


LINKS

Table of n, a(n) for n=0..9.
R. J. Mathar, OEIS A306892
R. J. Mathar, 2regular digraphs of the Lovelock Lagrangian, arXiv:1903.12477 (2019)


EXAMPLE

On n=1 node the graph is the node with two edges looping back to the node. On n=2 nodes, the graph is either having two pairs of edges (4 edges in total) linking one node to the other, or a loop at each node and two edges (different senses) from one node to the other.


CROSSREFS

Cf. A306827 (no loops).
Sequence in context: A298361 A000109 A049338 * A115275 A000679 A266932
Adjacent sequences: A306889 A306890 A306891 * A306893 A306894 A306895


KEYWORD

nonn,more


AUTHOR

R. J. Mathar, Mar 15 2019


EXTENSIONS

a(8) added.  R. J. Mathar, Apr 08 2019
a(9) added.  R. J. Mathar, Apr 15 2019


STATUS

approved



