OFFSET
1,1
COMMENTS
Numbers of the form p^8 (8th row of A120458) or p^2*r^2 (A085986), where p and r are distinct primes. - R. J. Mathar, Mar 01 2010
LINKS
R. J. Mathar, Table of n, a(n) for n = 1..1000
FORMULA
A000005(a(n)) = 9. - Juri-Stepan Gerasimov, Oct 10 2009
Sum_{n>=1} 1/a(n) = (P(2)^2 - P(4))/2 + P(8) = 0.0678286..., where P is the prime zeta function. - Amiram Eldar, Jul 03 2022
MATHEMATICA
Select[Range[90000], DivisorSigma[0, #]==9&] (* Vladimir Joseph Stephan Orlovsky, May 05 2011 *)
PROG
(PARI) is(n)=numdiv(n)==9 \\ Charles R Greathouse IV, Jun 19 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved