OFFSET
1,3
COMMENTS
Formally defined as number of isomorphism classes of connected multigraphs free from cut edges and generalized loops, with all vertices of degree four except for four external vertices of degree one.
LINKS
Klara Bragesjö Björkén and Kasper Johansson, Feynman Graphs.
EXAMPLE
a(1) = 1 [=o=] ;
a(2) = 1 [=o=o=] ;
a(3) = 2 [=o=o=o= and -o--o=]
||/
-o
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Kasper Johansson, Aug 20 2025
STATUS
approved
