A139118 Numbers with a nonprime number of divisors.

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

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

Amiram Eldar, Table of n, a(n) for n = 1..10000

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

Cf. A000005, A009087, A018252.

Sequence in context: A368459 A050199 A268388 * A089229 A345945 A123240

Adjacent sequences: A139115 A139116 A139117 * A139119 A139120 A139121

Keyword

easy,nonn

Author

Omar E. Pol, May 09 2008

Extensions

Extended by Ray Chandler, Jun 25 2009

Status

approved