OFFSET
1,2
FORMULA
G.f.: Sum_{k>0, k == 0, 2 or 5 mod 7} k * x^k/(1 - x^k).
MATHEMATICA
a[n_] := DivisorSum[n, # &, MemberQ[{0, 2, 5}, Mod[#, 7]] &]; Array[a, 100] (* Amiram Eldar, Jun 23 2023 *)
PROG
(PARI) my(N=90, x='x+O('x^N)); concat(0, Vec(sum(k=1, N, [1, 0, 1, 0, 0, 1, 0][k%7+1]*k*x^k/(1-x^k))))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Seiichi Manyama, Jun 23 2023
STATUS
approved