OFFSET
1,2
FORMULA
G.f.: Sum_{k>0} x^k/(1 - x^(3*k-2)).
MATHEMATICA
a[n_] := DivisorSum[3*n-2, 1 &, Mod[#, 3] == 1 &]; Array[a, 100] (* Amiram Eldar, Aug 23 2023 *)
PROG
(PARI) a(n) = sumdiv(3*n-2, d, d%3==1);
(PARI) my(N=100, x='x+O('x^N)); Vec(sum(k=1, N, x^k/(1-x^(3*k-2))))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Seiichi Manyama, Dec 21 2022
STATUS
approved