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Primes p such that 2^p-1 has exactly 2 prime factors.
7

%I #11 Sep 29 2018 09:48:12

%S 11,23,37,41,59,67,83,97,101,103,109,131,137,139,149,167,197,199,227,

%T 241,269,271,281,293,347,373,379,421,457,487,523,727,809,881,971,983,

%U 997,1061,1063

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

%C a(40)>=1277. - _Amiram Eldar_, Sep 29 2018

%t 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

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

%K nonn,more

%O 1,1

%A _Artur Jasinski_, Dec 09 2007

%E a(17)-a(37) from _Arkadiusz Wesolowski_, Jan 26 2012

%E a(38)-a(39) from _Amiram Eldar_, Sep 29 2018