%I #23 Mar 31 2024 13:53:52
%S 2,3,5,7,13,17,19,31,61,127
%N Primes p such that 2^p - 1 is a Chen prime.
%C For p in this sequence, 2^p - 1 is called a Mersenne-Chen prime.
%C Conjecture: 2^127 - 1 is the largest Mersenne-Chen prime.
%C Except for the initial term 2, this sequence is the intersection of A000043 and A000978 given by A107360. - _Max Alekseyev_, Oct 28 2008, Jan 28 2010
%e a(5)=13 because 2^13 - 1 = 8191 is prime and 2^13 + 1 = 3*2731 is semiprime.
%t Select[Prime[Range[40]],PrimeQ[2^#-1]&&PrimeOmega[2^#+1]<3&] (* _James C. McMahon_, Mar 30 2024 *)
%Y Cf. A000043, A000978, A107360, A109611.
%K hard,more,nonn
%O 1,1
%A _Jason Earls_, Aug 15 2005