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A307180
Isomorphism classes of 2-in 2-out regular digraphs, allowing parallel edges but not loops.
1
1, 0, 1, 2, 6, 15, 68, 310, 2096, 16554, 152497, 1574652, 17947667, 223013311
OFFSET
0,4
LINKS
Richard J. Mathar, 2-regular Digraphs of the Lovelock Lagrangian, arXiv:1903.12477 [math.GM], 2019.
CROSSREFS
Cf. A306827 (connected, inverse Eul. Trans.)
Sequence in context: A216811 A351900 A335751 * A009455 A244443 A346555
KEYWORD
nonn,hard,more
AUTHOR
R. J. Mathar, Mar 28 2019
STATUS
approved