OFFSET
1,4
LINKS
Jean-François Alcover, Table of n, a(n) for n = 1..40
EXAMPLE
a(3)=1: the non-edge joins the two leaves. a(4)=6: quadrangle: the non-edge is a diagonal; triangle with protruding edge: the non-edge joins the leaf with a node of degree 2; quadrangle with diagonal: the non-edge is the other diagonal; tetrahedron: no contribution; linear chain: the non-edge either joins the two leaves or a leaf with a node at distance 2; star graph: the non-edge joins two leaves.
MATHEMATICA
CROSSREFS
KEYWORD
nonn
AUTHOR
Brendan McKay, May 05 2018
STATUS
approved