OFFSET
1,2
LINKS
Michael De Vlieger, Table of n, a(n) for n = 1..10000
FORMULA
G.f.: -Sum_{k>=1} (-x)^k/(1-x^k)^3 = Sum_{k>=1} binomial(k+1,2) * x^k/(1+x^k).
MATHEMATICA
Table[DivisorSum[n, (-1)^(n/# - 1)*Binomial[# + 1, 2] &], {n, 56}] (* Michael De Vlieger, Oct 25 2023 *)
PROG
(PARI) a(n) = sumdiv(n, d, (-1)^(n/d-1)*binomial(d+1, 2));
CROSSREFS
KEYWORD
nonn,look
AUTHOR
Seiichi Manyama, Oct 24 2023
STATUS
approved