This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A033942 At least 3 prime factors (counted with multiplicity). 20
 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; text; internal format)
 OFFSET 1,1 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, Dec 29 2001 A001222(a(n)) > 3; A054576(a(n)) > 1. - Reinhard Zumkeller, Mar 10 2006 Also numbers such that no permutation of all divisors exists with coprime adjacent elements: A109810(a(n))=0. - Reinhard Zumkeller, May 24 2010 A211110(a(n)) > 2. - Reinhard Zumkeller, Apr 02 2012 A060278(a(n)) > 0. - Reinhard Zumkeller, Apr 05 2013 Volumes of rectangular cuboids with each side > 1. - Peter Woodward, Jun 16 2015 LINKS T. D. Noe, Table of n, a(n) for n = 1..1000 FORMULA Product p_i^e_i with Sum e_i >= 3. a(n) ~ n. - Charles R Greathouse IV, May 04 2013 MAPLE with(numtheory): A033942:=n->`if`(bigomega(n)>2, n, NULL): seq(A033942(n), n=1..200); # Wesley Ivan Hurt, Jun 23 2015 MATHEMATICA Select[ Range[150], Plus @@ Last /@ FactorInteger[ # ] > 2 &] (* Robert G. Wilson v, Oct 12 2005 *) Select[Range[150], PrimeOmega[#]>2&] (* Harvey P. Dale, Jun 22 2011 *) PROG (Haskell) a033942 n = a033942_list !! (n-1) a033942_list = filter ((> 2) . a001222) [1..] -- Reinhard Zumkeller, Oct 27 2011 (PARI) is(n)=bigomega(n)>2 \\ Charles R Greathouse IV, May 04 2013 CROSSREFS Cf. A014612. A101040(a(n))=0. A033987 is a subsequence; complement of A037143. - Reinhard Zumkeller, May 24 2010 Subsequence of A080257. Sequence in context: A152758 A160258 A071280 * A111087 A166083 A091375 Adjacent sequences:  A033939 A033940 A033941 * A033943 A033944 A033945 KEYWORD nonn,nice AUTHOR EXTENSIONS Corrected by Patrick De Geest, Jun 15 1998 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified February 21 10:49 EST 2019. Contains 320372 sequences. (Running on oeis4.)