OFFSET
1,65
FORMULA
G.f.: Sum_{k>0} (-1)^(k-1) * x^(6*k-1) / (1 - x^(6*k-1)).
a(n) = Sum_{d|n, d==5 (mod 6)} (-1)^((d-5)/6).
MATHEMATICA
a[n_] := DivisorSum[n, (-1)^((#-5)/6) &, Mod[#, 6] == 5 &]; Array[a, 100] (* Amiram Eldar, Jul 03 2023 *)
PROG
(PARI) a(n) = sumdiv(n, d, (d%6==5)*(-1)^((d-5)/6));
CROSSREFS
KEYWORD
sign
AUTHOR
Seiichi Manyama, Jul 03 2023
STATUS
approved