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a(n) is the denominator of Sum_{d|n, d < sqrt(n)} 1/d.
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%I #7 May 18 2024 02:23:19

%S 1,1,1,1,1,2,1,2,1,2,1,6,1,2,3,2,1,6,1,4,3,2,1,12,1,2,3,4,1,30,1,4,3,

%T 2,5,12,1,2,3,20,1,1,1,4,15,2,1,4,1,10,3,4,1,1,5,28,3,2,1,20,1,2,21,4,

%U 5,1,1,4,3,70,1,8,1,2,15,4,7,1,1,40

%N a(n) is the denominator of Sum_{d|n, d < sqrt(n)} 1/d.

%F Denominators of coefficients in expansion of Sum_{k>=1} x^(k*(k+1)) / (k * (1 - x^k)).

%e 0, 1, 1, 1, 1, 3/2, 1, 3/2, 1, 3/2, 1, 11/6, 1, 3/2, 4/3, 3/2, 1, 11/6, ...

%t nmax = 80; CoefficientList[Series[Sum[x^(k (k + 1))/(k (1 - x^k)), {k, 1, nmax}], {x, 0, nmax}], x] // Denominator // Rest

%o (PARI) a(n) = denominator(sumdiv(n, d, if (d^2 < n, 1/d))); \\ _Michel Marcus_, May 14 2024

%Y Cf. A017666, A070039, A372834 (numerators).

%K nonn,frac

%O 1,6

%A _Ilya Gutkovskiy_, May 14 2024