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 A137623 #5 Mar 30 2012 18:50:59 %S A137623 1,2,3,4,8,5,10,6,12,14,9,16,18,20,11,24,25,7,13,27,28,15,30,32,33,34, %T A137623 36,19,21,38,39,40,22,44,45,23,48,50,51,26,52,55,56,57,29,58,31,62,63, %U A137623 64,17,65,68,35,70,72,37,74,75,77,78,80,41,42,84,85,43,86,87,90,91,46 %N A137623 A137621(A137621(n)). %H A137623 <a href="/index/Per#IntegerPermutation">Index entries for sequences that are permutations of the natural numbers</a> %K A137623 nonn %O A137623 1,2 %A A137623 _Reinhard Zumkeller_, Jan 30 2008