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A139118 Numbers with a nonprime number of divisors. 4
1, 6, 8, 10, 12, 14, 15, 18, 20, 21, 22, 24, 26, 27, 28, 30, 32, 33, 34, 35, 36, 38, 39, 40, 42, 44, 45, 46, 48, 50, 51, 52, 54, 55, 56, 57, 58, 60, 62, 63, 65, 66, 68, 69, 70, 72, 74, 75, 76, 77, 78, 80, 82, 84, 85, 86, 87, 88, 90, 91, 92, 93, 94, 95, 96, 98, 99, 100, 102 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
A000005(a(n)) is nonprime. Complement of A009087. Also, nonprime numbers with nonprime number of divisors.
The sequence consists of those n such that n is not a prime power, or n of the form p^k where k+1 is composite. - Franklin T. Adams-Watters, Apr 09 2009
LINKS
EXAMPLE
15 is in the sequence because it has 4 divisors: 1, 3, 5, and 15. - Emeric Deutsch, Jun 27 2009
MAPLE
with(numtheory): a := proc (n) if isprime(tau(n)) = false then n else end if end proc: seq(a(n), n = 1 .. 120); # Emeric Deutsch, Jun 27 2009
MATHEMATICA
Select[Range[102], ! PrimeQ[DivisorSigma[0, #]] &] (* Amiram Eldar, Nov 27 2020 *)
PROG
(PARI) is(n)=!isprime(numdiv(n)) \\ Charles R Greathouse IV, Jun 19 2016
CROSSREFS
Sequence in context: A368459 A050199 A268388 * A089229 A345945 A123240
KEYWORD
easy,nonn
AUTHOR
Omar E. Pol, May 09 2008
EXTENSIONS
Extended by Ray Chandler, Jun 25 2009
STATUS
approved

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Last modified March 19 01:57 EDT 2024. Contains 370952 sequences. (Running on oeis4.)