A307180 Isomorphism classes of 2-in 2-out regular digraphs, allowing parallel edges but not loops.

1, 0, 1, 2, 6, 15, 68, 310, 2096, 16554, 152497, 1574652, 17947667, 223013311

Offset: 0

Author

R. J. Mathar, Mar 28 2019

Keywords

• nonn
• hard
• more

Links

• Richard J. Mathar, 2-regular Digraphs of the Lovelock Lagrangian, arXiv:1903.12477 [math.GM], 2019.

Crossrefs

Cf. A306827 (connected, inverse Eul. Trans.)