OFFSET
1,3
LINKS
Seiichi Manyama, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = (1/3) * Sum_{d | 3*n-1, d==1 (mod 3)} (d+2).
G.f.: Sum_{k>0} x^k / (1 - x^(3*k-1))^2.
MATHEMATICA
a[n_] := DivisorSum[3*n - 1, # + 2 &, Mod[#, 3] == 1 &]/3; Array[a, 100] (* Amiram Eldar, Jul 05 2023 *)
PROG
(PARI) a(n) = sumdiv(3*n-1, d, (d%3==1)*(d+2))/3;
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jul 04 2023
STATUS
approved