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
KEYWORD
sign
AUTHOR
Seiichi Manyama, Jul 01 2023
STATUS
approved