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 A307155 #6 Mar 27 2019 11:14:18 %S A307155 1,0,0,1,2,5,22,90,616,4988,46883,493406,5712809,71909602 %N A307155 Number of connected, loopless 2-regular digraphs, no multiedges, on n nodes. %C A307155 Inverse Euler transform of A219889. %C A307155 The Multiset Transform (number of loopless 2-regular digraphs on n nodes with c weakly connected components) of this sequence starts %C A307155 in row 3 as: %C A307155 1 ; %C A307155 2 ; %C A307155 5 ; %C A307155 22 , 1 ; %C A307155 90 , 2 ; %C A307155 616 , 8 ; %C A307155 4988 , 32 , 1 ; %C A307155 46883 , 149 , 2 ; %C A307155 493406 , 906 , 8 ; %C A307155 5712809 , 6923 , 36 , 1 ; %C A307155 71909602 , 61919 , 164 , 2 ; %Y A307155 Cf. A219889 (not necessarily connected), A005642 (loops admitted). %K A307155 nonn %O A307155 0,5 %A A307155 _R. J. Mathar_, Mar 27 2019