OFFSET
1,2
FORMULA
a(n) = -Sum_{d|n, n/d==1 (mod 3)} (-1)^(n/d) * d^2.
MATHEMATICA
a[n_] := -DivisorSum[n, (-1)^(n/#) * #^2 &, Mod[n/#, 3] == 1 &]; Array[a, 100] (* Amiram Eldar, Jul 01 2023 *)
PROG
(PARI) a(n) = -sumdiv(n, d, (n/d%3==1)*(-1)^(n/d)*d^2);
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jul 01 2023
STATUS
approved