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A192690 Nonprime numbers with a nonprime number of nonprime divisors. 1
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 (list; graph; refs; listen; history; text; internal format)
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 * A064499 A080258 A054290

Adjacent sequences:  A192687 A192688 A192689 * A192691 A192692 A192693

KEYWORD

nonn

AUTHOR

Juri-Stepan Gerasimov, Oct 15 2011

STATUS

approved

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Last modified July 17 20:45 EDT 2019. Contains 325109 sequences. (Running on oeis4.)