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 A005642 M2689 #24 May 12 2022 17:12:24 %S A005642 1,3,7,24,117,663,4824,40367,381554,4001849,46043780,576018785, %T A005642 7783281188,112953364381,1752128923245,28930230194371,506596534953769 %N A005642 Number of 2-diregular connected digraphs with n nodes. %D A005642 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). %H A005642 M. V. S. Ramanath & 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) %H A005642 R. J. Mathar, <a href="/A005642/a005642.pdf">Illustrations</a> %Y A005642 Cf. A005641 (not necess. connected), A306892 (multiarcs allowed). Labeled version is A123544. %K A005642 nonn %O A005642 2,2 %A A005642 _N. J. A. Sloane_