OFFSET
1,2
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..10000
FORMULA
a(A004611(n)) = 0.
G.f.: Sum_{k>=1} x^(2*k)*(2+3*x^k+x^(3*k))/(1-x^(3*k))^2. - Vladeta Jovovic, Apr 11 2006
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[(#-1)/3]&]]; Array[sd, 80] (* Harvey P. Dale, May 04 2011 *)
PROG
(PARI) for(n=1, 100, print1(sumdiv(n, d, if(d%3!=1, d))", "))
(PARI)
N = 66; x = 'x + O('x^N);
gf = sum(n=1, N, (3*n-1)*x^(3*n-1)/(1-x^(3*n-1)) + (3*n)*x^(3*n)/(1-x^(3*n)) );
v = Vec(gf); concat([0], v)
\\ Joerg Arndt, May 17 2013
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Ralf Stephan, Apr 02 2003
STATUS
approved