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 A107721 #3 Mar 30 2012 18:36:46 %S A107721 1,2,13,131,1741,28451,549757,12247211,308953453,8706012827, %T A107721 271093643293,9245857326635,342832527086797,13733511532751099, %U A107721 591127774090746493,27209560375187822795,1333804962202136755501 %N A107721 Column 1 of triangle A107719. %o A107721 (PARI) %Y A107721 Cf. A107717, A107719. %K A107721 nonn %O A107721 0,2 %A A107721 _Paul D. Hanna_, May 30 2005