OFFSET
1,2
FORMULA
G.f.: Sum_{k>0} (-1)^(k-1) * x^(4*k-3) / (1 - x^(4*k-3))^2.
a(n) = Sum_{d|n, d==1 (mod 4)} (-1)^((d-1)/4) * (n/d).
MATHEMATICA
a[n_] := DivisorSum[n, (-1)^((# - 1)/4) * n/# &, Mod[#, 4] == 1 &]; Array[a, 100] (* Amiram Eldar, Jul 03 2023 *)
PROG
(PARI) a(n) = sumdiv(n, d, (d%4==1)*(-1)^((d-1)/4)*n/d);
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jul 01 2023
STATUS
approved