OFFSET
1,3
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..10000
FORMULA
a(A003627(n)) = 1.
G.f.: Sum_{k>=1} x^k*(1 + 3*x^(2*k) + 2*x^(3*k))/(1 - x^(3*k))^2. - Ilya Gutkovskiy, Sep 12 2019
Sum_{k=1..n} a(k) ~ c * n^2, where c = Pi^2/18 = 0.548311... (A086463). - Amiram Eldar, Jan 06 2024
MATHEMATICA
sd[n_]:= Total[Select[Divisors[n], !IntegerQ[(# - 2) / 3]&]]; Array[sd, 100] (* Vincenzo Librandi, May 17 2013 *)
PROG
(PARI) for(n=1, 100, print1(sumdiv(n, d, if(d%3!=2, d))", "))
(PARI)
N = 66; x = 'x + O('x^N);
gf = sum(n=1, N, (3*n-2)*x^(3*n-2)/(1-x^(3*n-2)) + (3*n)*x^(3*n)/(1-x^(3*n)) );
v = Vec(gf)
\\ Joerg Arndt, May 17 2013
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Ralf Stephan, Apr 02 2003
STATUS
approved