prove that  2 ⁿ - 1 is a prime

Hi!

2ⁿ – 1 can be a prime number in some cases like, for n = 2, 2ⁿ – 1 = 2² – 1 = 4 – 1 = 3 which is prime.

 

But for n = 4, 2⁴ – 1 = 16 – 1 = 15 which is not prime.

 

Cheers