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 A131138 #4 Jul 15 2014 10:18:06 %S A131138 0,1,2,3,3,4,5,5,6,7,6,7,8,8,9,10,10,11,12,11,12,13,13,14,15,15,16,17, %T A131138 15,16,17,17,18,19,19,20,21,20,21,22,22,23,24,24,25,26,25,26,27,27,28, %U A131138 29,29,30,31,29,30,31,31,32,33,33,34,35,34,35,36,36 %N A131138 a(n)=log_3(A131137(n)). %o A131138 (PARI) a(n)=if(n<4,n,valuation(denominator(polylog(1-n,1/4)/4),3)) \\ _Charles R Greathouse IV_, Jul 15 2014 %K A131138 easy,nonn %O A131138 0,3 %A A131138 _Paul Barry_, Jun 17 2007