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A105441 Numbers with at least two odd prime factors (not necessarily distinct). 7
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

Complement of A093641; A087436(a(n)) > 1; A093642 is a subsequence.

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

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

Sequence in context: A037002 A071149 A110473 * A093642 A177733 A207675

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 December 10 15:13 EST 2016. Contains 279003 sequences.