OFFSET
1,4
LINKS
Seiichi Manyama, Table of n, a(n) for n = 1..10000
FORMULA
G.f.: Sum_{k>0} (-1)^(k-1) * x^(3*k-1) / (1 - x^(3*k-1))^2.
a(n) = Sum_{d|n, n/d==2 (mod 3)} (-1)^(n/d) * d.
MATHEMATICA
a[n_] := DivisorSum[n, (-1)^(n/#) * # &, Mod[n/#, 3] == 2 &]; Array[a, 100] (* Amiram Eldar, Jul 01 2023 *)
PROG
(PARI) a(n) = sumdiv(n, d, (n/d%3==2)*(-1)^(n/d)*d);
CROSSREFS
KEYWORD
sign
AUTHOR
Seiichi Manyama, Jul 01 2023
STATUS
approved