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 A170921 #16 Oct 04 2019 22:22:20
%S A170921 3,45,105,4725,66825,127702575,383107725,18091198125,1856156927625,
%T A170921 183759535834875,9056719980433125,275344870859667984375,
%U A170921 1298054391195577640625,3952575621190533915703125,367589532770719654160390625,6249022057102234120726640625,3842566358093920359949921875
%N A170921 Write x*cot(x) = Product_{n>=1} (1 + g_n*x^(2*n)); a(n) = denominator(g_n).
%e A170921 -1/3, -1/45, -1/105, -16/4725, -91/66825, -58844/127702575, -73267/383107725, ...
%p A170921 t1:=x*cot(x);
%p A170921 L:=100;
%p A170921 t0:=series(t1, x, L);
%p A170921 g:=[];
%p A170921 M:=20; # number of terms to get
%p A170921 t2:=t0:
%p A170921 for n from 1 to M do
%p A170921 t3:=coeff(t2, x, 2*n); t2:=series(t2/(1+t3*x^(2*n)), x, L); g:=[op(g), t3];
%p A170921 od:
%p A170921 g;
%p A170921 g1:=map(numer, g);
%p A170921 g2:=map(denom, g);
%Y A170921 Cf. A170920.
%K A170921 nonn,frac
%O A170921 1,1
%A A170921 _N. J. A. Sloane_, Jan 31 2010
%E A170921 Corrected definition and terms - _N. J. A. Sloane_, Oct 04 2019 (thanks to _Petros Hadjicostas_ for pointing out that something was wrong).