OFFSET
1,4
LINKS
Seiichi Manyama, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = Sum_{d|n, n/d==2 (mod 3)} (-1)^(d+1) * d.
MATHEMATICA
nmax = 80; CoefficientList[Series[Sum[x^(3 k + 2)/(1 + x^(3 k + 2))^2, {k, 0, nmax}], {x, 0, nmax}], x] // Rest
Table[DivisorSum[n, (-1)^(# + 1) # &, MemberQ[{2}, Mod[n/#, 3]] &], {n, 1, 80}]
CROSSREFS
KEYWORD
sign
AUTHOR
Ilya Gutkovskiy, Jul 14 2023
STATUS
approved