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 A357556 #9 Oct 08 2022 15:11:16
%S A357556 1,4,9,16,25,6,49,64,81,50,121,72,169,98,45,256,289,108,361,200,441,
%T A357556 242,529,288,625,338,729,392,841,15,961,1024,1089,578,49,1296,1369,
%U A357556 722,1521,800,1681,147,1849,88,2025,1058,2209,128,2401,2500,2601,1352,2809,243,3025
%N A357556 a(n) is the denominator of Sum_{d|n} (-1)^(d+1) / d^2.
%F A357556 Denominators of coefficients in expansion of Sum_{k>=1} (-1)^(k+1) * x^k / (k^2 * (1 - x^k)).
%e A357556 1, 3/4, 10/9, 11/16, 26/25, 5/6, 50/49, 43/64, 91/81, 39/50, 122/121, ...
%t A357556 Table[Sum[(-1)^(d + 1)/d^2, {d, Divisors[n]}], {n, 1, 55}] // Denominator
%t A357556 nmax = 55; CoefficientList[Series[Sum[(-1)^(k + 1) x^k/(k^2 (1 - x^k)), {k, 1, nmax}], {x, 0, nmax}], x] // Denominator // Rest
%o A357556 (Python)
%o A357556 from sympy import divisors
%o A357556 from fractions import Fraction
%o A357556 def a(n): return sum(Fraction((-1)**(d+1), d*d) for d in divisors(n, generator=True)).denominator
%o A357556 print([a(n) for n in range(1, 56)]) # _Michael S. Branicky_, Oct 03 2022
%o A357556 (PARI) a(n) = denominator(sumdiv(n, d, (-1)^(d+1)/d^2)); \\ _Michel Marcus_, Oct 03 2022
%Y A357556 Cf. A017668, A064027, A098988, A321543, A334580, A357555 (numerators).
%K A357556 nonn,frac
%O A357556 1,2
%A A357556 _Ilya Gutkovskiy_, Oct 03 2022