OFFSET
1,4
FORMULA
a(n) = (1/4) * Sum_{d | 4*n-1, d==1 (mod 4)} (d+3).
G.f.: Sum_{k>0} x^k / (1 - x^(4*k-1))^2.
MATHEMATICA
a[n_] := DivisorSum[4*n - 1, # + 3 &, Mod[#, 4] == 1 &]/4; Array[a, 100] (* Amiram Eldar, Jul 05 2023 *)
PROG
(PARI) a(n) = sumdiv(4*n-1, d, (d%4==1)*(d+3))/4;
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jul 04 2023
STATUS
approved