%I #9 May 12 2019 17:52:57
%S 1,0,1,2,6,15,68,310,2096,16554,152497,1574652,17947667,223013311
%N Isomorphism classes of 2-in 2-out regular digraphs, allowing parallel edges but not loops.
%H Richard J. Mathar, <a href="https://arxiv.org/abs/1903.12477">2-regular Digraphs of the Lovelock Lagrangian</a>, arXiv:1903.12477 [math.GM], 2019.
%Y Cf. A306827 (connected, inverse Eul. Trans.)
%K nonn,hard,more
%O 0,4
%A _R. J. Mathar_, Mar 28 2019