A twin prime is a prime number that differs from another prime number by two. Except for the pair (2, 3), this is the smallest possible difference between two primes. Some examples of twin prime pairs are 5 and 7, 11 and 13, and 821 and 823. (Sometimes the term twin prime is used for a pair of twin primes; an alternative name for this is prime twin.)
The question of whether there exist infinitely many twin primes has been one of the great open questions in number theory for many years. This is the content of the Twin Prime Conjecture. A strong form of the Twin Prime Conjecture, the Hardy-Littlewood conjecture, postulates a distribution law for twin primes akin to the prime number theorem.
Using his celebrated sieve method, Brun shows that the number of twin primes less than x is << x/(log x)2. This result implies that the sum of the reciprocals of all twin primes converges (see Brun's constant). This is in stark contrast to the sum of the reciprocals of all primes, which diverges.
He also shows that every even number can be represented in infinitely many ways as a difference of two numbers both having at most 9 prime factors. Chen Jing Run's well known theorem states that for any m even, there are infinitely many primes that differs by m from a number having at most two prime factors.
(Before Brun attacked to the twin prime problem, Merlin had also attempted to solve this problem using sieve method. But unfortunately he was killed in WWI)
Every twin prime pair greater than 3 is of the form (6n - 1, 6n + 1) for some natural numbern, and with the exception of n = 1, n must end in 0, 2, 3, 5, 7, or 8.
It has been proven that the pair m, m + 2 is a twin prime if and only if
As of 2004, the largest known twin prime is 33218925 · 2169690 ± 1; it was found in 2002 by Papp using the free Proth and NewPGen software.
