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The twin prime conjecture is a famous problem in number theory that involves prime numbers. It was first proposed by Euclid around 300 B.C.^{[citation needed]} and states:

There are infinitely many primes p such that p + 2 is also prime.

Such a pair of prime numbers is called a prime twin. The conjecture has been researched by many number theorists. Mathematicians believe the conjecture to be true, based only on numerical evidence and heuristic reasoning involving the probabilistic distribution of primes using Cramér's model.

In 1849 de Polignac made the more general conjecture that for every natural number k, there are infinitely many prime pairs p and p′ such that p - p′ = 2k. The case k = 1 is the twin prime conjecture.

Contents 1 Partial results 2 Hardy–Littlewood conjecture 3 See also 4 References 5 External links

In 1915, Viggo Brun showed that the sum of reciprocals of the twin primes was convergent. This famous result, called Brun's theorem was the first use of the Brun sieve and helped initiate the development of modern sieve theory. The modern version of Brun's argument can be used to show that the number of twin primes less than N does not exceed

for some absolute constant C > 0.

In 1940, Paul Erdős showed that there is a constant c < 1 and infinitely many primes p such that (p′ - p) < (c ln p) where p′ denotes the next prime after p. This result was successively improved; in 1986 Helmut Maier showed that a constant c < 0.25 can be used. In 2004 Daniel Goldston and Cem Yıldırım showed that the constant could be improved further to c = 0.085786… In 2005, Goldston, János Pintz and Yıldırım established that c can be chosen to be arbitrarily small ^[1][2]

In fact, by assuming the Elliott-Halberstam conjecture, they were able to show that there are infinitely many n such that at least two of n, n + 2, n + 6, n + 8, n + 12, n + 18, or n + 20 are prime.

In 1966, Chen Jingrun showed that there are infinitely many primes p such that p + 2 is either a prime or a semiprime (i.e., the product of two primes). The approach he took involved sieve theory, and he managed to treat the twin prime conjecture and Goldbach's conjecture in similar manners.

Defining a Chen prime to be a prime p such that p + 2 is either a prime or a semiprime, Terence Tao and Ben Green showed in 2005 that there are infinitely many three-term arithmetic progressions of Chen primes.

The Hardy–Littlewood conjecture (after G. H. Hardy and John Littlewood) is a generalization of the twin prime conjecture. It is concerned with the distribution of prime constellations, including twin primes, in analogy to the prime number theorem. Let π₂(x) denote the number of primes p ≤ x such that p + 2 is also prime. Define the twin prime constant C₂ as^[3]

(here the product extends over all prime numbers p ≥ 3). Then the conjecture is that

in the sense that the quotient of the two expressions tends to 1 as x approaches infinity.

This conjecture can be justified (but not proven) by assuming that

describes the density function of the prime distribution, an assumption suggested by the prime number theorem.

• Twin prime
• Cramér's conjecture

1. ^ Small Gaps between Primes Exist (article abstract) (2007). Retrieved on 2007-06-20.
2. ^ Small gaps between primes or almost primes (article abstact) (2007). Retrieved on 2007-06-20.
3. ^ A page of number theoretical constants (2007). Retrieved on 2007-06-20.

• NOVA Science Now on the Twin Prime Conjecture
• Terrence Tao on the difficulty of proving the twin primes conjecture
• Eric W. Weisstein, Twin Prime Conjecture at MathWorld.
• Eric W. Weisstein, Twin Primes Constant at MathWorld.