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 A181993 #28 Jun 18 2019 06:15:48
%S A181993 1,2,6,15,630,2835,155925,6081075,1277025750,10854718875,
%T A181993 1856156927625,194896477400625,2900518163668125,3698160658676859375,
%U A181993 1298054391195577640625,263505041412702261046875,245059688513813102773593750,4043484860477916195764296875
%N A181993 Denominator of (4^n*(4^n-1)/2)*B_{2n}/(2n)!, B_{n} Bernoulli number.
%C A181993 Numerator is (-1)^(n+1)*A046990(n).
%H A181993 Michel Marcus, <a href="/A181993/b181993.txt">Table of n, a(n) for n = 0..100</a>
%H A181993 William Rowan Hamilton, <a href="https://doi.org/10.1080/14786444308644751">On an expression for the numbers of Bernoulli, by means of a definite integral, and on some connected processes of summation and integration</a>, Philosophical Magazine, 23 (1843), pp. 360-367.
%F A181993 a(n) = denominator of (1/Pi)*Integral(x>=0, (sin(x)/x)^(2*n)*sin(2*n*x)*tan(x)).
%p A181993 A181993 := n -> denom((4^n*(4^n-1)/2)*bernoulli(2*n)/(2*n)!);
%p A181993 seq(A181993(i), i=0..18);
%t A181993 a[n_] := Denominator[4^n (4^n-1)/2 BernoulliB[2n]/(2n)!];
%t A181993 Table[a[n], {n, 0, 17}] (* _Jean-François Alcover_, Jun 18 2019 *)
%o A181993 (PARI) a(n) = denominator((4^n*(4^n-1)/2)*bernfrac(2*n)/(2*n)!); \\ _Michel Marcus_, Jun 18 2019
%Y A181993 Cf. A046990.
%K A181993 nonn,frac
%O A181993 0,2
%A A181993 _Peter Luschny_, Apr 05 2012