The final digit of all prime numbers is either: 1,2,3,5,7,9
Of those, 1 and 9 do not appear as single digit primes...
And 2 and 5 only appear as single digit primes.
Also, of the four digit primes, the final 3 digits can be any of those "final primes" listed above except 2 and 5, as long as the first digit is a 1 or a 2, with the exception of 1333.

As of January 2017 , the largest known prime number is 274,207,281 − 1, a number with 22,338,618 digits.

newolder wrote:As of January 2017 , the largest known prime number is 274,207,281 − 1, a number with 22,338,618 digits.

That's a Mersene prime if I'm not mistaken?
Is every number calculated via that rule of (2^n)-1 prime?

Eratosthenes sieve is another method and there are probably others.


See also, Prime (Ulam) spirals: