

A173978


Numbers n such that the least prime factor of 2n  3 is less than that of 2n  1, unless 2n  3 and 2n  1 are (twin) primes.


2



2, 6, 9, 12, 15, 18, 19, 21, 24, 27, 30, 33, 34, 36, 39, 40, 42, 45, 48, 49, 51, 54, 57, 60, 61, 63, 64, 66, 69, 72, 75, 78, 79, 81, 82, 84, 87, 90, 93, 94, 96, 99, 102, 105, 106, 108, 109, 111, 112, 114, 117, 120, 123, 124
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OFFSET

1,1


COMMENTS

Integers > 1 for which 2n  3 is not in A001359 and A020639(2n3)< A020639(2n1).
Every multiple of 3 greater than 3 is in the sequence.


LINKS

Table of n, a(n) for n=1..54.


EXAMPLE

a(3) = 9 because 2*9  3 = 15, the least prime factor of which is 3 and that is smaller than the least prime factor of 2*9  1 = 17.


MATHEMATICA

Select[Range[200], Not[PrimeQ[2#  3] && PrimeQ[2#  1]] && TrueQ[FactorInteger[2#  3][[1, 1]] < FactorInteger[2#  1][[1, 1]]] &] (* Alonso del Arte, Jun 05 2011 *)


CROSSREFS

Cf. A020639, A001359, A173977.
Sequence in context: A171504 A287445 A119720 * A000134 A120701 A189752
Adjacent sequences: A173975 A173976 A173977 * A173979 A173980 A173981


KEYWORD

nonn


AUTHOR

Vladimir Shevelev, Mar 04 2010


EXTENSIONS

More terms from Alonso del Arte, Jun 05 2011


STATUS

approved



