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 A157968 #2 Mar 31 2012 10:24:25 %S A157968 0,1,15,63,127,255,511,1023,4095,16383,32767,65535,262143,1048575, %T A157968 2097151,4194303,16777215,33554431,67108863,134217727,268435455, %U A157968 1073741823,4294967295 %N A157968 2^n - 1 (A000225) where n is nonprime number (A141468). %C A157968 a(0) = 0 since 2^0 - 1 = 0 and 0 is nonprime, a(1) = 1 since 2^1 - 1 = 1 and 1 is nonprime, a(2) = 15 since 2^4 - 1 = 15 and 4 is nonprime, ... %Y A157968 A000668 %K A157968 nonn %O A157968 0,3 %A A157968 _Jani Melik_, Mar 10 2009