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A105441 Numbers with at least two odd prime factors (not necessarily distinct). 14
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.

Cf. A001227, A087436, A138591.

Sequence in context: A289689 A316752 A110473 * A325164 A093642 A177733

Adjacent sequences:  A105438 A105439 A105440 * A105442 A105443 A105444

KEYWORD

nonn,easy

AUTHOR

Reinhard Zumkeller, Apr 09 2005

STATUS

approved

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Last modified January 26 22:05 EST 2022. Contains 350601 sequences. (Running on oeis4.)