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%I M2757 #26 May 12 2019 18:22:08
%S 1,3,8,27,131,711,5055,41607,389759,4065605,46612528,581713045,
%T 7846380548,113718755478,1762208816647,29073392136390,508777045979418
%N Number of 2-diregular digraphs with n nodes.
%C These graphs may have loops, whereas A219889 does not admit them. Both sequences do not admit multiedges. - _R. J. Mathar_, Mar 27 2019
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%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.
%H M. V. S. Ramanath and T. R. Walsh, <a href="/A005641/a005641.pdf">Enumeration and generation of a class of regular digraphs</a>, J. Graph Theory 11 (1987), no. 4, 471-479. (Annotated scanned copy)
%Y Cf. A005642 (inverse Euler transform, connected graphs).
%K nonn
%O 2,2
%A _N. J. A. Sloane_
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