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 A340563 #16 Mar 18 2023 05:51:48
%S A340563 1,1,2,16,384,30976,7741440,6369316864,16435095011328,
%T A340563 138915523039657984,3696387867279360000000,
%U A340563 321533678904455375050768384,88192375153215003517412966400000,78996127242669742603293261855977373696,223311937686075869460797609709638544686841856
%N A340563 a(n) = sqrt( Product_{1<=j, k<=n-1} (4*sin(j*Pi/n)^2 + 4*cos(k*Pi/n)^2) ).
%F A340563 a(n) ~ c * (sqrt(2) - 1)^n * exp(2*G*n^2/Pi), where c = sqrt(Pi) / Gamma(3/4)^2 if n is even and c = 2^(1/4) if n is odd, G is Catalan's constant A006752. - _Vaclav Kotesovec_, Mar 18 2023
%t A340563 Table[Sqrt[Product[Product[(4*Sin[j*Pi/n]^2 + 4*Cos[k*Pi/n]^2), {j, 1, n - 1}], {k, 1, n - 1}]], {n, 0, 15}] // Round (* _Vaclav Kotesovec_, Mar 18 2023 *)
%o A340563 (PARI) default(realprecision, 120);
%o A340563 {a(n) = round(sqrt(prod(j=1, n-1, prod(k=1, n-1, 4*sin(j*Pi/n)^2+4*cos(k*Pi/n)^2))))}
%Y A340563 Main diagonal of A340561.
%Y A340563 Cf. A127606, A340562.
%K A340563 nonn
%O A340563 0,3
%A A340563 _Seiichi Manyama_, Jan 11 2021