A135979 Indices n such that 2^prime(n)-1 has exactly 2 distinct prime factors.

5, 9, 12, 13, 17, 19, 23, 25, 26, 27, 29, 32, 33, 34, 35, 39, 45, 46, 49, 53, 57, 58, 60, 62, 69, 74, 75, 82, 88, 93, 99, 129, 140, 152, 164, 166, 168, 178, 179

Offset

1,1

Comments

a(40)>=206. - Amiram Eldar, Sep 29 2018

Links

Table of n, a(n) for n=1..39.

Formula

Equals {k: A001221(A001348(k)) = 2}. a(n) = A049084(A135978(n)). - R. J. Mathar, May 03 2008

Mathematica

k = {}; Do[If[ ! PrimeQ[2^Prime[n] - 1], c = FactorInteger[2^Prime[n] - 1]; d = Length[c]; If[d == 2, AppendTo[k, n]]], {n, 1, 40}]; k
Select[Range[40], PrimeNu[2^Prime[#]-1]==2&] (* Harvey P. Dale, Jul 07 2013 *)

Crossrefs

Cf. A000225, A065341, A054723, A134852, A135975, A135976, A135977, A135978.

Sequence in context: A314609 A190691 A314610 * A287452 A287456 A123574

Adjacent sequences: A135976 A135977 A135978 * A135980 A135981 A135982

Keyword

nonn,more

Author

Artur Jasinski, Dec 09 2007

Extensions

Edited by R. J. Mathar, May 03 2008

a(17)-a(34) from Donovan Johnson, Jun 14 2009

a(35)-a(39) from Amiram Eldar, Sep 29 2018

Status

approved