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A105441 Numbers with at least two odd prime factors (not necessarily distinct). 15
9, 15, 18, 21, 25, 27, 30, 33, 35, 36, 39, 42, 45, 49, 50, 51, 54, 55, 57, 60, 63, 65, 66, 69, 70, 72, 75, 77, 78, 81, 84, 85, 87, 90, 91, 93, 95, 98, 99, 100, 102, 105, 108, 110, 111, 114, 115, 117, 119, 120, 121, 123, 125, 126, 129, 130, 132, 133, 135, 138, 140, 141 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Also polite numbers (A138591) that can be expressed as the sum of two or more consecutive integers in more than one ways. For example 9=4+5 and 9=2+3+4. Also 15=7+8, 15=4+5+6 and 15=1+2+3+4+5. - Jayanta Basu, Apr 30 2013
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
FORMULA
A087436(a(n)) > 1.
A001227(a(n)) > 2. [Reinhard Zumkeller, May 01 2012]
MATHEMATICA
opf3Q[n_]:=Count[Flatten[Table[First[#], {Last[#]}]&/@FactorInteger[n]], _?OddQ]>1 (* Harvey P. Dale, Jun 13 2011 *)
PROG
(PARI) upTo(lim)=my(v=List(), p=7, m); forprime(q=8, lim, forstep(n=p+2, q-2, 2, m=n; while(m<=lim, listput(v, m); m<<=1)); p=q); forstep(n=p+2, lim, 2, listput(v, n)); vecsort(Vec(v)) \\ Charles R Greathouse IV, Aug 08 2011
(PARI) is(n)=n>>=valuation(n, 2); !isprime(n) && n>1 \\ Charles R Greathouse IV, Apr 30 2013
(Haskell)
a105441 n = a105441_list !! (n-1)
a105441_list = filter ((> 2) . a001227) [1..]
-- Reinhard Zumkeller, May 01 2012
CROSSREFS
Complement of A093641; A093642 is a subsequence.
Sequence in context: A358725 A358576 A110473 * A325164 A093642 A364566
KEYWORD
nonn,easy
AUTHOR
Reinhard Zumkeller, Apr 09 2005
STATUS
approved

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Last modified March 19 04:26 EDT 2024. Contains 370952 sequences. (Running on oeis4.)