Question 13443
How about 5, 13, or 23?  There are a TON of examples of primes p such 2p-1 is NOT a prime.


5*2-1 = 9 which is NOT a prime.
2*13 - 1 = 25 which is NOT a prime.
2*23 - 1 = 45 which is NOT a prime.


R^2 at SCC