A135978 Primes p such that 2^p-1 has exactly 2 prime factors.

11, 23, 37, 41, 59, 67, 83, 97, 101, 103, 109, 131, 137, 139, 149, 167, 197, 199, 227, 241, 269, 271, 281, 293, 347, 373, 379, 421, 457, 487, 523, 727, 809, 881, 971, 983, 997, 1061, 1063

Offset: 1

Author

Artur Jasinski, Dec 09 2007

Keywords

• nonn
• more

Comments

a(40)>=1277. - Amiram Eldar, Sep 29 2018

Crossrefs

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

Programs

Mathematica

k = {}; Do[If[ ! PrimeQ[2^Prime[n] - 1], c = FactorInteger[2^Prime[n] - 1]; d = Length[c]; If[d == 2, AppendTo[k, Prime[n]]]], {n, 1, 40}]; k

Extensions

a(17)-a(37) from Arkadiusz Wesolowski, Jan 26 2012

a(38)-a(39) from Amiram Eldar, Sep 29 2018