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 A117139 #3 Mar 30 2012 16:50:24 %S A117139 0,0,1,3,0,2,5,1,6,0,3,7,2,8,1,9,0,4,9,3,10,2,11,1,12,0,5,11,4,12,3, %T A117139 13,2,14,1,15,0,6,13,5,14,4,15,3,16,2,17,1,18,0 %N A117139 Similar to A117137 but begin each row with 0. Equals A117137 - 1. %K A117139 nonn,tabf %O A117139 0,4 %A A117139 _N. J. A. Sloane_, Apr 21 2006