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 A213680 #23 Mar 05 2013 18:02:43
%S A213680 6,21,291,56451,2124476931,3008934820234119171,
%T A213680 6035792501611554034238453484153151491,
%U A213680 24287194081673507672666338605180770497437885188248737771493963111463682051
%N A213680 a(n) = 2*a(n-1)^2/3-3 with a(0)=6.
%F A213680 a(n) = 3*cosh(2^n*arccosh(2)).
%F A213680 a(n) = 3*A002812(n). [_Giovanni Resta_, Mar 04 2013]
%e A213680 a(1) = 2*a(0)^2/3-3 = 2*6^2/3-3 = 21,
%e A213680 a(2) = 2*a(1)^2/3-3 = 2*21^2/3-3 = 291,
%e A213680 a(3) = 2*a(2)^2/3-3 = 2*291^2/3-3 = 2*84681/3-3 = 56451.
%e A213680 Or, by the first formula:
%e A213680 a(3) = 3*cosh(2^3*arccosh(2)) = 56451,
%e A213680 a(4) = 3*cosh(2^4*arccosh(2)) = 2124476931.
%o A213680 (Sage)
%o A213680 var('x')
%o A213680 p=2*x^2/3-3
%o A213680 s=[6]
%o A213680 for i in [0..8]:
%o A213680 s=s+[p(s[i])]
%o A213680 show(s)
%Y A213680 Cf. A002812.
%K A213680 nonn
%O A213680 0,1
%A A213680 _Jose Eduardo Blazek_, Mar 04 2013