OFFSET

0,2

COMMENTS

a(0) = 1 by convention. Loops add two to the degree of a node.

a(n) is also the number of connected multigraphs on 2*n+1 unlabeled nodes with one vertex of degree 2 and all others of degree 3, loops allowed.

EXAMPLE

The a(1) = 3 multigraphs correspond to either a triple edge rooted on one of the edges or to a single edge with a loop at each end and rooted on either the edge or the loop.

CROSSREFS

KEYWORD

nonn

AUTHOR

Andrew Howroyd, Mar 12 2023

STATUS

approved