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 A090648 #15 Jul 28 2015 12:50:06
%S A090648 0,1,1,3,17,31,3,5461,257,657,6355,60787,12291,22369621,617093,149943,
%T A090648 16843009,5726623061,71613,91625968981,162529435,4870483401,
%U A090648 50991843607,499069107643,12129927891,34118178995231,5664905191661,45148868444817,165649641466501
%N A090648 a(n)=2*(4^n-1)/denominator(B(2n)) where B(k) denotes the k-th Bernoulli number.
%t A090648 Table[(2*(4^n-1))/Denominator[BernoulliB[2n]],{n,0,40}] (* _Harvey P. Dale_, Jul 20 2011 *)
%o A090648 (PARI) a(n)=2*(4^n-1)/denominator(bernfrac(2*n))
%Y A090648 Cf. A001469.
%K A090648 nonn
%O A090648 0,4
%A A090648 _Benoit Cloitre_, Dec 13 2003
%E A090648 Corrected and extended by _Harvey P. Dale_, Jul 20 2011