OFFSET
1,1
COMMENTS
For p in this sequence, 2^p - 1 is called a Mersenne-Chen prime.
Conjecture: 2^127 - 1 is the largest Mersenne-Chen prime.
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
EXAMPLE
a(5)=13 because 2^13 - 1 = 8191 is prime and 2^13 + 1 = 3*2731 is semiprime.
MATHEMATICA
Select[Prime[Range[40]], PrimeQ[2^#-1]&&PrimeOmega[2^#+1]<3&] (* James C. McMahon, Mar 30 2024 *)
CROSSREFS
KEYWORD
hard,more,nonn
AUTHOR
Jason Earls, Aug 15 2005
STATUS
approved