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 A116213 #3 Mar 31 2012 13:20:26
%S A116213 1,3,3855,
%T A116213 450552876409790643671482431940419874915447411150352389258589821042463539455
%N A116213 (2^(2^(2^n))-1)/(2^(2^n)+1).
%C A116213 2^n+1 divides 2^(2^n)-1 iff n is a power of 2.
%F A116213 a(n) = (2^(2^(2^n))-1)/(2^(2^n)+1). a(n) = A051179(2^n)/A000215(n).
%t A116213 Table[ (2^2^2^n - 1) / (2^2^n + 1), {n,0,3} ]
%Y A116213 Cf. A000215 = Fermat numbers: 2^(2^n)+1. Cf. A051179 = 2^(2^n)-1.
%K A116213 nonn
%O A116213 0,2
%A A116213 _Alexander Adamchuk_, Apr 08 2007