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 A031239 #4 Mar 30 2012 18:56:21 %S A031239 3,11,21,26,28,30,31,32,34,41,54,69,84,91,94,97,99,100,103,114,129, %T A031239 144,159,166,169,172,174,175,178,189,197,200,203,204,206,209,212,215, %U A031239 218,219,221,224,227,230,233,234,236,239,241,242 %N A031239 Position of n-th 3 in A031235. %K A031239 nonn %O A031239 1,1 %A A031239 _Clark Kimberling_