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 A139474 #14 Nov 28 2017 06:41:00
%S A139474 1,35,53789260175,300027707381709879256191290532493317737977820735
%N A139474 a(n) = ((2*sqrt(2) + 3)^(2^(prime(n) - 1) - 1) - (3 - 2*sqrt(2))^(2^(prime(n) - 1) - 1))/(4*sqrt(2)).
%C A139474 Next term a(5) has 783 decimal digits.
%C A139474 Conjecture of _Kenneth J Ramsey_ from May 16 2006 (see A001109): a(n) is divisible by 2^prime(n)-1 if and only 2^prime(n)-1 is a Mersenne prime.
%t A139474 Table[Expand[((2*Sqrt[2] + 3)^(2^(Prime[n] - 1) - 1) - (3 - 2*Sqrt[2])^(2^(Prime[n] - 1) - 1))/(4*Sqrt[2])], {n, 1, 10}]
%Y A139474 Cf. A001109, A139474, A139475, A139476.
%K A139474 nonn
%O A139474 1,2
%A A139474 _Artur Jasinski_, Apr 22 2008