login
This site is supported by donations to The OEIS Foundation.

 

Logo

Annual Appeal: Today, Nov 11 2014, is the 4th anniversary of the launch of the new OEIS web site. 70,000 sequences have been added in these four years, all edited by volunteers. Please make a donation (tax deductible in the US) to help keep the OEIS running.

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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

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

Content is available under The OEIS End-User License Agreement .

Last modified December 21 23:06 EST 2014. Contains 252326 sequences.