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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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Integers > 1 for which 2n - 3 is not in A001359 and A020639(2n-3)< A020639(2n-1).

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

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Last modified August 20 05:25 EDT 2019. Contains 326139 sequences. (Running on oeis4.)