OFFSET
1,1
COMMENTS
Maple implementation: see A030513.
Numbers of the form p^20 or p^2*q^6 (A189990) where p and q are distinct primes. - R. J. Mathar, Mar 01 2010
LINKS
T. D. Noe, Table of n, a(n) for n = 1..1000
FORMULA
A000005(a(n)) = 21.
Sum_{n>=1} 1/a(n) = P(2)*P(6) - P(8) + P(20) = 0.00365945..., where P is the prime zeta function. - Amiram Eldar, Jul 03 2022
MATHEMATICA
Select[Range[450000], DivisorSigma[0, #]==21&] (* Vladimir Joseph Stephan Orlovsky, May 03 2011 *)
PROG
(PARI) is(n)=numdiv(n)==21 \\ Charles R Greathouse IV, Jun 19 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
R. J. Mathar, Apr 22 2008
STATUS
approved