A363628 - OEIS

A363628

Expansion of Sum_{k>0} (1/(1-x^k)^3 - 1).

3, 9, 13, 24, 24, 47, 39, 69, 68, 96, 81, 153, 108, 165, 170, 222, 174, 292, 213, 342, 302, 363, 303, 523, 375, 492, 474, 615, 468, 766, 531, 783, 686, 810, 726, 1101, 744, 999, 938, 1248, 906, 1402, 993, 1413, 1306, 1437, 1179, 1901, 1314, 1773, 1562, 1938, 1488, 2238, 1698

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OFFSET

1,1

LINKS

Table of n, a(n) for n=1..55.

FORMULA

G.f.: Sum_{k>0} binomial(k+2,2) * x^k/(1 - x^k).

a(n) = Sum_{d|n} binomial(d+2,2).

MATHEMATICA

a[n_] := DivisorSum[n, Binomial[# + 2, 2] &]; Array[a, 50] (* Amiram Eldar, Jul 05 2023 *)

PROG

(PARI) a(n) = sumdiv(n, d, binomial(d+2, 2));