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A364016
Expansion of Sum_{k>0} k^2 * x^(2*k) / (1 + x^(3*k)).
2
0, 1, 0, 4, -1, 9, 0, 17, 0, 21, -1, 36, 0, 50, -9, 68, -1, 81, 0, 85, 0, 117, -1, 153, -25, 170, 0, 200, -1, 189, 0, 273, -9, 285, -50, 324, 0, 362, 0, 365, -1, 450, 0, 469, -81, 525, -1, 612, 0, 526, -9, 680, -1, 729, -146, 850, 0, 837, -1, 765, 0, 962, 0, 1092, -170, 1053, 0, 1141, -9
OFFSET
1,4
FORMULA
a(n) = Sum_{d|n, n/d==2 (mod 3)} (-1)^(n/d) * d^2.
MATHEMATICA
a[n_] := DivisorSum[n, (-1)^(n/#) * #^2 &, Mod[n/#, 3] == 2 &]; Array[a, 100] (* Amiram Eldar, Jul 03 2023 *)
PROG
(PARI) a(n) = sumdiv(n, d, (n/d%3==2)*(-1)^(n/d)*d^2);
CROSSREFS
Sequence in context: A141680 A141681 A176215 * A143469 A360610 A331147
KEYWORD
sign
AUTHOR
Seiichi Manyama, Jul 01 2023
STATUS
approved