Twin primes
From Conservapedia
Twin primes are pairs of prime numbers separated by only 2.
The smallest twin primes are (3, 5), (5, 7), (11, 13), (17, 19), (29, 31), (41, 43).
All twin primes except (3, 5) are of the form 6n +/- 1.[1]
Proving that there are infinitely many twin primes is one of the great unsolved problems in mathematics.
