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, 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

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

Amiram Eldar, Table of n, a(n) for n = 1..1000

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 A350235

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