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.
Table of n, a(n) for n=0..5.
Richard J. Mathar, Feynman diagrams of the QED vacuum polarization, vixra:1901.0148 (2019) Section II.
Cf. A005967 (unsigned), A054499 (only one cycle of pluses), A170946 (directed plus-edges).
Sequence in context: A326563 A316700 A124348 * A268605 A205804 A052324
Adjacent sequences: A323386 A323387 A323388 * A323390 A323391 A323392
R. J. Mathar, Jan 13 2019