This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A005641 M2757 #26 May 12 2019 18:22:08 %S A005641 1,3,8,27,131,711,5055,41607,389759,4065605,46612528,581713045, %T A005641 7846380548,113718755478,1762208816647,29073392136390,508777045979418 %N A005641 Number of 2-diregular digraphs with n nodes. %C A005641 These graphs may have loops, whereas A219889 does not admit them. Both sequences do not admit multiedges. - _R. J. Mathar_, Mar 27 2019 %D A005641 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). %H A005641 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 A005641 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 A005641 Cf. A005642 (inverse Euler transform, connected graphs). %K A005641 nonn %O A005641 2,2 %A A005641 _N. J. A. Sloane_