OFFSET
0,2
COMMENTS
Obtained from the cubic graphs A005967 (connected undirected cubic graphs that may have loops and/or multiedges) by signing each edge with a plus or a minus such that two pluses and one minus meet at each vertex.
LINKS
Richard J. Mathar, Feynman diagrams of the QED vacuum polarization, vixra:1901.0148 (2019) Section II.
PROG
(PARI) \\ See A339645 for combinatorial species functions.
cycleIndexSeries(n)={1+sLog(sCartProd(sExp(dihedralGroupSeries(n)), sExp(symGroupCycleIndex(2)*x^2 + O(x*x^n))))}
seq(n)={Vec(substpol(OgfSeries(cycleIndexSeries(2*n)), x^2, x))} \\ Andrew Howroyd, May 05 2023
CROSSREFS
KEYWORD
nonn
AUTHOR
R. J. Mathar, Jan 13 2019
EXTENSIONS
Terms a(6) and beyond from Andrew Howroyd, May 05 2023
STATUS
approved