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 A334580 #21 May 09 2020 03:31:28
%S A334580 1,4,36,144,3600,1200,58800,235200,6350400,6350400,768398400,
%T A334580 768398400,129859329600,129859329600,129859329600,519437318400,
%U A334580 150117385017600,50039128339200,18064125330451200,3612825066090240,3612825066090240,3612825066090240,1911184459961736960
%N A334580 Denominator of Sum_{k=1..n} (-1)^(k+1)/k^2.
%C A334580 For n = 1 to n = 19, we have a(n) = A068589(n), but a(20) = 3612825066090240 <> A068589(20) = 18064125330451200.
%e A334580 The first few fractions are 1, 3/4, 31/36, 115/144, 3019/3600, 973/1200, 48877/58800, 191833/235200, 5257891/6350400, 5194387/6350400, ... = A119682/A334580.
%p A334580 b := proc(n) local k: add((-1)^(k + 1)/k^2, k = 1 .. n): end proc:
%p A334580 seq(denom(b(n)), n=1..30);
%o A334580 (PARI) a(n) = denominator(sum(k=1, n, (-1)^(k+1)/k^2)); \\ _Michel Marcus_, May 07 2020
%Y A334580 Cf. A068589, A119682 (numerators).
%K A334580 nonn,frac
%O A334580 1,2
%A A334580 _Petros Hadjicostas_, May 06 2020