# Twin prime

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Twin prime

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 (3, 5), (5, 7), (11, 13), (17, 19), (29, 31) and (41, 43). Sometimes the term twin prime is used for a pair of twin primes; an alternative name for this is prime twin.

 Are there infinitely many twin primes?

## History

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

A stronger form of the twin prime conjecture, the Hardy–Littlewood conjecture, postulates a distribution law for twin primes akin to the prime number theorem.

## Brun's theorem

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

$\frac{CN}{(\log{N})^2}$

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]

$\liminf_{n\to\infty}\frac{p_{n+1}-p_n}{\log p_n}=0.$

In fact, by assuming the Elliott–Halberstam conjecture or a slightly weaker version, 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. Under a stronger hypothesis they showed that for infinitely many n at least two of n, n + 2, n + 4, and n + 6 are prime.

Every twin prime pair except (3, 5) is of the form (6n − 1, 6n + 1) for some natural number n, 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

$4((m-1)! + 1) \equiv -m \pmod {m(m+2)}.$

If m − 4 or m + 6 is also prime then the 3 primes are called a prime triplet.

## Largest known twin prime pair

On January 15, 2007 two distributed computing projects, Twin Prime Search and PrimeGrid found the largest known twin primes, 2003663613 · 2195000 ± 1. The numbers have 58711 decimal digits. Their discoverer was Eric Vautier of France.

On August 6, 2009 those same two projects announced that a new record twin prime had been found.[3] It is 65516468355 · 2333333 ± 1.[4] The numbers have 100355 decimal digits.

An empirical analysis of all prime pairs up to 4.35 · 1015 shows that if the number of such pairs less than x is f(xx/(log x)2 then f(x) is about 1.7 for small x and decreases towards about 1.3 as x tends to infinity.

There are 808,675,888,577,436 twin prime pairs below 1018.[5]

The limiting value of f(x) is conjectured to equal twice the twin prime constant (not to be confused with Brun's constant)

$2 \prod_{\textstyle{p\;{\rm prime}\atop p \ge 3}} \left(1 - \frac{1}{(p-1)^2}\right) = 1.3203236\ldots;$

(sequence A114907 in OEIS) this conjecture would imply the twin prime conjecture, but remains unresolved.

The twin prime conjecture would give a better approximation, as with the prime counting function, by

$\pi_2(x) \approx 2C_2\; \operatorname{li}_2(x) = 2C_2 \int_2^x \frac{dt}{\left(\log_e t \right)^2}.$

## Properties

The first few twin prime pairs are:

(3, 5), (5, 7), (11, 13), (17, 19), (29, 31), (41, 43), (59, 61), (71, 73), (101, 103), (107, 109), (137, 139), … (sequence A077800 in OEIS).

Since every third odd number is divisible by 3, no three successive odd numbers can be prime unless one of them is 3, thus 5 is the only prime which is part of two pairs. Also, along the same lines, other than the first pair, the number centered between the twin primes must always be divisible by 6. The lower member of a pair is by definition a Chen prime.

## First Hardy–Littlewood conjecture

The Hardy–Littlewood conjecture (after G. H. Hardy and John Littlewood) is a generalization of the twin prime conjecture.[citation needed] 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 px such that p + 2 is also prime. Define the twin prime constant C2 as[6]

$C_2 = \prod_{p\ge 3} \frac{p(p-2)}{(p-1)^2} \approx 0.66016 18158 46869 57392 78121 10014\dots$

(sequence A005597 in OEIS) (here the product extends over all prime numbers p ≥ 3). Then the conjecture is that

$\pi_2(n) \sim 2 C_2 \frac{n}{(\ln n)^2} \sim 2 C_2 \int_2^n {dt \over (\ln t)^2}$

in the sense that the quotient of the two expressions tends to 1 as n approaches infinity. (The second ~ is not part of the conjecture and is proved by integration by parts.)

This conjecture can be justified (but not proven) by assuming that 1 / ln t describes the density function of the prime distribution, an assumption suggested by the prime number theorem.

## Polignac's conjecture

Polignac's conjecture from 1849 states that for every even natural number k, there are infinitely many consecutive prime pairs p and p′ such that p′ − p = 2k (i.e. there are infinitely many prime gaps of size 2k). The case k = 1 is the twin prime conjecture. The conjecture has not been proved or disproved for any value of k.

## Isolated prime

An isolated prime is a prime number p such that neither p − 2 nor p + 2 is prime. In other words, p is not part of a twin prime pair. For example, 23 is an isolated prime since 21 and 25 are both composite.

The first few isolated primes are

2, 23, 37, 47, 53, 67, 79, 83, 89, 97 … (sequence A007510 in OEIS).

## References

1. ^ Goldston, Daniel Alan; Motohashi, Yoichi; Pintz, János; Yıldırım, Cem Yalçın (2006), "Small gaps between primes exist", Japan Academy. Proceedings. Series A. Mathematical Sciences 82 (4): 61–65, arXiv:math.NT/0505300, MR2222213 .
2. ^ Goldston, D. A.; Graham, S. W.; Pintz, J.; Yıldırım, C. Y. (2009), "Small gaps between primes or almost primes", Transactions of the American Mathematical Society 361 (10): 5285–5330, arXiv:math.NT/0506067, doi:10.1090/S0002-9947-09-04788-6, MR2515812 .
3. ^ "News Archive". PrimeGrid. 6 August 2009. Retrieved 2009-08-07.
4. ^ "The Prime Database: 65516468355*2^333333-1". Prime Pages. 13 August 2009. Retrieved 2009-08-14.
5. ^ Tomás Oliveira e Silva (7 April 2008). "Tables of values of pi(x) and of pi2(x)". Aveiro University. Retrieved 7 January 2011.
6. ^ "A page of number theoretical constants". 2007. Retrieved 2011-02-02.

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