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 A134721 #2 Mar 30 2012 17:33:50 %S A134721 3,31,8191,524287,2305843009213693951, %T A134721 162259276829213363391578010288127 %N A134721 Bisection of Mersenne primes A000668. %F A134721 a(n) = A000668(2*n - 1). %e A134721 a(3) = 8191 because the (2*3-1=5)th Mersenne prime is 8191. %Y A134721 Cf. A000668, A134722. %K A134721 easy,nonn %O A134721 1,1 %A A134721 _Omar E. Pol_, Nov 11 2007