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A306892 Isomorphism classes of connected 2-regular digraphs on n nodes, allowing multiarcs and loops. 3
1, 1, 2, 5, 14, 50, 265, 1601, 11984, 101884 (list; graph; refs; listen; history; text; internal format)
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, 2-regular 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

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Last modified November 21 04:33 EST 2019. Contains 329350 sequences. (Running on oeis4.)