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A033942
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At least 3 prime factors (counted with multiplicity).
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12
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8, 12, 16, 18, 20, 24, 27, 28, 30, 32, 36, 40, 42, 44, 45, 48, 50, 52, 54, 56, 60, 63, 64, 66, 68, 70, 72, 75, 76, 78, 80, 81, 84, 88, 90, 92, 96, 98, 99, 100, 102, 104, 105, 108, 110, 112, 114, 116, 117, 120, 124, 125, 126, 128, 130, 132, 135, 136, 138, 140, 144
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| See also A002808 for 'Composite numbers' or 'Divisible by at least 2 primes'.
A001055(a(n)) > 2, e.g. for a(3)=18 there are 4 factorizations: 1*18 = 2*9 = 2*3*3 = 3*6. - Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Dec 29, 2001
A001222(a(n)) > 3; A054576(a(n)) > 1. - Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Mar 10 2006
Also numbers such that no permutation of all divisors exists with coprime adjacent elements: A109810(a(n))=0. [From Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), May 24 2010]
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LINKS
| T. D. Noe, Table of n, a(n) for n=1..1000
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FORMULA
| Product p_i^e_i with Sum e_i >= 3.
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MATHEMATICA
| Select[ Range[150], Plus @@ Last /@ FactorInteger[ # ] > 2 &] (from Robert G. Wilson v (rgwv(at)rgwv.com), Oct 12 2005)
Select[Range[150], PrimeOmega[#]>2&] (* From Harvey P. Dale, June 22 2011 *)
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PROG
| (Haskell)
a033942 n = a033942_list !! (n-1)
a033942_list = filter ((> 2) . a001222) [1..]
-- Reinhard Zumkeller, Oct 27 2011
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CROSSREFS
| Cf. A014612.
A101040(a(n))=0.
A033987 is a subsequence; complement of A037143. [From Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), May 24 2010]
Sequence in context: A160258 A071280 * A111087 A166083 A091375 A192991
Adjacent sequences: A033939 A033940 A033941 * A033943 A033944 A033945
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KEYWORD
| nonn,nice
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AUTHOR
| Jeff Burch (gburch(AT)erols.com)
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EXTENSIONS
| Corrected by Patrick De Geest (pdg(AT)worldofnumbers.com), Jun 15 1998.
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