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 A320420 #10 Jan 17 2019 16:11:12 %S A320420 0,0,1,6,33,227,2235,31558,1668992 %N A320420 Number of monoids of order n that are not groups. %F A320420 a(n) = A058129(n) - A000001(n). %Y A320420 Cf. A058129, A000001. %K A320420 nonn,more %O A320420 0,4 %A A320420 _Steve Szabo_, Jan 07 2019