A192690 Nonprime numbers with a nonprime number of nonprime divisors.

1, 12, 16, 18, 20, 24, 28, 40, 44, 45, 48, 50, 52, 54, 56, 60, 63, 64, 68, 72, 75, 76, 80, 81, 84, 88, 90, 92, 96, 98, 99, 104, 108, 112, 116, 117, 124, 126, 132, 135, 136, 140, 147, 148, 150, 152, 153, 156, 160, 162, 164, 171, 172, 175, 176, 180, 184, 188

Offset

1,2

Links

Table of n, a(n) for n=1..58.

Example

For example, 12 is composite and it has 6 divisors: 1, 2, 3, 4, 6, 12. Of these, 4 are not prime: 1, 4, 6, 12. Since 4 is not prime either, 12 is in the sequence.

Mathematica

NonPrimeDivisors[n_] := Length[Select[Divisors[n], ! PrimeQ[#] &]]; Select[Range[200], ! PrimeQ[#] && ! PrimeQ[NonPrimeDivisors[#]] &] (* T. D. Noe, Oct 20 2011 *)

Crossrefs

Cf. A139118, A141468, A167175.

Cf. A018252. - Omar E. Pol, Oct 20 2011

Sequence in context: A058080 A119911 A272861 * A378082 A064499 A080258

Adjacent sequences: A192687 A192688 A192689 * A192691 A192692 A192693

Keyword

nonn

Author

Juri-Stepan Gerasimov, Oct 15 2011

Status

approved