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
KEYWORD
nonn,easy
AUTHOR
Reinhard Zumkeller, Apr 09 2005
STATUS
approved