OFFSET
1,3
COMMENTS
Number of divisors of n that are not of the form 3*k + 2.
FORMULA
Sum_{k=1..n} a(k) ~ 2*n*log(n)/3 + c*n, where c = (5*gamma-2)/3 - gamma(2,3) = (5*A001620-2)/3 - A256843 = 0.222152... . - Amiram Eldar, Jan 14 2024
MATHEMATICA
nmax = 90; CoefficientList[Series[Sum[x^k (1 + x^(2 k))/(1 - x^(3 k)), {k, 1, nmax}], {x, 0, nmax}], x] // Rest
Table[DivisorSum[n, 1 &, !MemberQ[{2}, Mod[#, 3]] &], {n, 1, 90}]
PROG
(PARI) a(n) = {numdiv(n) - sumdiv(n, d, d%3==2)} \\ Andrew Howroyd, Sep 11 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Sep 11 2019
STATUS
approved