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A033987
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Numbers that are divisible by at least 4 primes (counted with multiplicity).
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15
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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)
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OFFSET
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1,1
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COMMENTS
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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
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LINKS
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T. D. Noe, Table of n, a(n) for n=1..1000
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FORMULA
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Product p_i^e_i with Sum e_i >= 4.
A001055(a(n)) > A033273(a(n)). - Juri-Stepan Gerasimov, Nov 09 2009
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MAPLE
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with(numtheory): A033987:=n->`if`(bigomega(n)>3, n, NULL): seq(A033987(n), n=1..300); # Wesley Ivan Hurt, May 26 2015
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MATHEMATICA
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Select[Range[300], PrimeOmega[#]>3&] (* Harvey P. Dale, Mar 20 2016 *)
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PROG
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(PARI) is(n)=bigomega(n)>3 \\ Charles R Greathouse IV, May 26 2015
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CROSSREFS
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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
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KEYWORD
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nonn,easy
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AUTHOR
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N. J. A. Sloane
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EXTENSIONS
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More terms from Patrick De Geest, Jun 15 1998
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STATUS
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approved
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