[seqfan] almost 2-regular digraphs derived from A005642

[Richard J. Mathar]

In A005642 the connected digraphs with indegree=outdegree=2 at every
node are counted. Someone with a background in elementary particle
physics (like me) would obviously call graphs of that kind almost
irrelevant "because vacuum graphs cancel..." and move on to the
kind of binary encouter/collision graphs where 2 or 4 "free/external" nodes
indicate where the particles enter and exit. That sort of brainstorming
for graphs that are obtained by cutting arcs of the 2-regular digraphs
to that effect are collected in my PDF file https://oeis.org/A005642/a005642.pdf .
Whether there is any theory in physics which uses such a Fermi-type contact
interaction I don't know.

Richard Mathar