A030627 Numbers with 9 divisors.

36, 100, 196, 225, 256, 441, 484, 676, 1089, 1156, 1225, 1444, 1521, 2116, 2601, 3025, 3249, 3364, 3844, 4225, 4761, 5476, 5929, 6561, 6724, 7225, 7396, 7569, 8281, 8649, 8836, 9025, 11236, 12321, 13225, 13924, 14161, 14884, 15129

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

Cf. A000005, A030515, A030516, A030626, A085986, A120458, A179645.

Sequence in context: A303606 A303661 A175391 * A177492 A077448 A085986

Adjacent sequences: A030624 A030625 A030626 * A030628 A030629 A030630

Keyword

nonn

Author

Jeff Burch

Status

approved