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 A033987 Numbers that are divisible by at least 4 primes (counted with multiplicity). 15
 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 FrÃ¶hlich, 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 EXTENSIONS More terms from Patrick De Geest, Jun 15 1998 STATUS approved

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Last modified April 13 10:24 EDT 2021. Contains 342935 sequences. (Running on oeis4.)