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 A373440 #15 Jun 26 2024 06:02:22
%S A373440 1,1,1,4,1,1,1,4,9,1,1,4,1,1,1,16,1,9,1,4,1,1,1,4,25,1,9,4,1,1,1,16,1,
%T A373440 1,1,18,1,1,1,4,1,1,1,4,9,1,1,16,49,25,1,4,1,9,1,4,1,1,1,4,1,1,9,64,1,
%U A373440 1,1,4,1,1,1,18,1,1,25,4,1,1,1,16,81,1,1,4,1
%N A373440 Denominator of sum of reciprocals of square divisors of n.
%H A373440 Amiram Eldar, <a href="/A373440/b373440.txt">Table of n, a(n) for n = 1..10000</a>
%F A373440 Denominators of coefficients in expansion of Sum_{k>=1} x^(k^2)/(k^2*(1-x^(k^2))).
%F A373440 a(n) is the denominator of Sum_{d^2|n} 1/d^2.
%e A373440 1, 1, 1, 5/4, 1, 1, 1, 5/4, 10/9, 1, 1, 5/4, 1, 1, 1, 21/16, 1, 10/9, 1, 5/4, 1, 1, 1, 5/4, 26/25, ...
%t A373440 nmax = 85; CoefficientList[Series[Sum[x^(k^2)/(k^2 (1 - x^(k^2))), {k, 1, nmax}], {x, 0, nmax}], x] // Rest // Denominator
%t A373440 f[p_, e_] := (p^2 - p^(-2*Floor[e/2]))/(p^2-1); a[1] = 1; a[n_] := Denominator[Times @@ f @@@ FactorInteger[n]]; Array[a, 100] (* _Amiram Eldar_, Jun 26 2024 *)
%o A373440 (PARI) a(n) = denominator(sumdiv(n, d, if (issquare(d), 1/d))); \\ _Michel Marcus_, Jun 05 2024
%Y A373440 Cf. A007407, A007947, A017666, A017668, A035316, A332881, A373439 (numerators).
%K A373440 nonn,frac
%O A373440 1,4
%A A373440 _Ilya Gutkovskiy_, Jun 05 2024