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A137484 Numbers with 21 divisors. 7
576, 1600, 2916, 3136, 7744, 10816, 18225, 18496, 23104, 33856, 35721, 53824, 61504, 62500, 87616, 88209, 107584, 118336, 123201, 140625, 141376, 179776, 210681, 222784, 238144, 263169, 287296, 322624, 341056, 385641, 399424, 440896 (list; graph; refs; listen; history; text; internal format)
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
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
Cf. A000005, A030513, A030638 (20 divisors), A137485 (22 divisors), A189990.
Sequence in context: A235181 A268773 A250789 * A189990 A064254 A281242
KEYWORD
nonn
AUTHOR
R. J. Mathar, Apr 22 2008
STATUS
approved

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Last modified March 18 22:56 EDT 2024. Contains 370952 sequences. (Running on oeis4.)