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A033987 Numbers that are divisible by at least 4 primes (counted with multiplicity). 13
16, 24, 32, 36, 40, 48, 54, 56, 60, 64, 72, 80, 81, 84, 88, 90, 96, 100, 104, 108, 112, 120, 126, 128, 132, 135, 136, 140, 144, 150, 152, 156, 160, 162, 168, 176, 180, 184, 189, 192, 196, 198, 200, 204, 208, 210, 216, 220, 224, 225, 228, 232, 234, 240, 243 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Complement of A037144: A001222(a(n)) > 3; A117358(a(n)) > 1. - Reinhard Zumkeller, Mar 10 2006

Also numbers such that no permutation of all proper divisors exists with coprime adjacent elements: A178254(a(n)) = 0. - Reinhard Zumkeller, May 24 2010

Also, numbers that can be written as a product of at least two composites, i.e., admit a nontrivial factorization into composites. - Felix Fröhlich, Dec 22 2018

LINKS

T. D. Noe, Table of n, a(n) for n=1..1000

FORMULA

Product p_i^e_i with Sum e_i >= 4.

A001055(a(n)) > A033273(a(n)). - Juri-Stepan Gerasimov, Nov 09 2009

MAPLE

with(numtheory): A033987:=n->`if`(bigomega(n)>3, n, NULL): seq(A033987(n), n=1..300); # Wesley Ivan Hurt, May 26 2015

MATHEMATICA

Select[Range[300], PrimeOmega[#]>3&] (* Harvey P. Dale, Mar 20 2016 *)

PROG

(PARI) is(n)=bigomega(n)>3 \\ Charles R Greathouse IV, May 26 2015

CROSSREFS

Subsequence of A033942; A178212 is a subsequence.

Cf. A014613, A001055, A033273.

Sequence in context: A114415 A320141 A225903 * A140349 A088493 A074451

Adjacent sequences:  A033984 A033985 A033986 * A033988 A033989 A033990

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane

EXTENSIONS

More terms from Patrick De Geest, Jun 15 1998

STATUS

approved

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Last modified October 22 22:34 EDT 2019. Contains 328335 sequences. (Running on oeis4.)