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 A113221 #6 Mar 30 2012 18:50:53 %S A113221 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,21,23,19,25,18,27,20,29, %T A113221 22,26,28,24,30,31,32,33,41,35,43,45,37,47,39,49,34,42,36,44,46,38,48, %U A113221 40,61,51,63,53,65,67,55,69,57,58,59,60,50,62,64,52,81,54,83,56,85,71,87 %N A113221 A113220(A113220(n)). %C A113221 Inverse integer permutation of T000004; %C A113221 A113218(a(n)) = a(A113218(n)) = A113220(n). %H A113221 <a href="/index/Per#IntegerPermutation">Index entries for sequences that are permutations of the natural numbers</a> %K A113221 nonn %O A113221 0,3 %A A113221 _Reinhard Zumkeller_, Oct 18 2005