

A264828


Nonprimes that are not twice a prime.


6



1, 8, 9, 12, 15, 16, 18, 20, 21, 24, 25, 27, 28, 30, 32, 33, 35, 36, 39, 40, 42, 44, 45, 48, 49, 50, 51, 52, 54, 55, 56, 57, 60, 63, 64, 65, 66, 68, 69, 70, 72, 75, 76, 77, 78, 80, 81, 84, 85, 87, 88, 90, 91, 92, 93, 95, 96, 98, 99, 100, 102, 104
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OFFSET

1,2


COMMENTS

Except the first term of sequence that is 1, if n is in the sequence, k*n is in the sequence for all k > 1. So odd semiprimes (A046315) and numbers of the form 4*p (A001749) where p is prime are core subsequences which give the initial terms of arithmetic progressions in this sequence.  Altug Alkan, Nov 29 2015


LINKS

Robert Israel, Table of n, a(n) for n = 1..10000


MAPLE

Primes, Nonprimes:= selectremove(isprime, {$1..1000}):
sort(convert(Nonprimes minus map(`*`, Primes, 2), list)); # Robert Israel, Nov 30 2015


MATHEMATICA

Select[Range@ 104, And[! PrimeQ@ #, Or[PrimeOmega@ # != 2, OddQ@ #]] &] (* Michael De Vlieger, Nov 27 2015 *)
Select[Range@110, Nor[PrimeQ[#], PrimeQ[#/2]] &] (* Vincenzo Librandi, Jan 22 2016 *)


PROG

(PARI) print1(1, ", "); forcomposite(n=1, 1e3, if(n % 2 == 1  !isprime(n/2), print1(n, ", "))) \\ Altug Alkan, Dec 01 2015


CROSSREFS

Cf. A009188, A018252, A100484.
Sequence in context: A199635 A131864 A179443 * A080756 A304661 A189833
Adjacent sequences: A264825 A264826 A264827 * A264829 A264830 A264831


KEYWORD

nonn,easy


AUTHOR

Giovanni Teofilatto, Nov 26 2015


STATUS

approved



