Twin prime

A twin prime is a prime number that is either 2 less or 2 more than another prime number—for example, either member of the twin prime pair (17, 19) or (41, 43). In other words, a twin prime is a prime that has a prime gap of two. Sometimes the term twin prime is used for a pair of twin primes; an alternative name for this is prime twin or prime pair.

Twin primes become increasingly rare as one examines larger ranges, in keeping with the general tendency of gaps between adjacent primes to become larger as the numbers themselves get larger. However, it is unknown whether there are infinitely many twin primes (the so-called twin prime conjecture) or if there is a largest pair. The breakthrough work of Yitang Zhang in 2013, as well as work by James Maynard, Terence Tao and others, has made substantial progress towards proving that there are infinitely many twin primes, but at present this remains unsolved.

Properties

Usually the pair (2, 3) is not considered to be a pair of twin primes. Since 2 is the only even prime, this pair is the only pair of prime numbers that differ by one; thus twin primes are as closely spaced as possible for any other two primes.

The first several 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), ... OEIS: A077800.

Five is the only prime that belongs to two pairs, as every twin prime pair greater than (3, 5) is of the form $(6n-1,6n+1)$ for some natural number n; that is, the number between the two primes is a multiple of 6. As a result, the sum of any pair of twin primes (other than 3 and 5) is divisible by 12.

Brun's theorem

Main article: 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 fact, it is bounded above by $${\frac {8C_{2}N}{(\log N)^{2}}}\left[1+\operatorname {\mathcal {O}} \left({\frac {\log \log N}{\log N}}\right)\right],$$ where $C_{2}$ is the twin prime constant (slightly less than 2/3), given below.

Twin prime conjecture

Unsolved problem in mathematics:

Are there infinitely many twin primes?

(more unsolved problems in mathematics)

The twin prime conjecture claims that the set of prime pairs is infinite. 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 that 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 primes p such that p + 2k is also prime. The case k = 1 of de Polignac's conjecture is the twin prime conjecture.

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

On 17 April 2013, Yitang Zhang announced a proof that there exists an integer N that is less than 70 million, where there are infinitely many pairs of primes that differ by N. Zhang's paper was accepted in early May 2013. Terence Tao subsequently proposed a Polymath Project collaborative effort to optimize Zhang's bound.

One year after Zhang's announcement, the bound had been reduced to 246, where it remains. These improved bounds were discovered using a different approach that was simpler than Zhang's and was discovered independently by James Maynard and Terence Tao. This second approach also gave bounds for the smallest f (m) needed to guarantee that infinitely many intervals of width f (m) contain at least m primes. Moreover (see also the next section) assuming the Elliott–Halberstam conjecture and its generalized form, the Polymath Project wiki states that the bound is 12 and 6, respectively.

A strengthening of Goldbach’s conjecture, if proved, would also prove there is an infinite number of twin primes, as would the existence of Siegel zeroes.

Theorems weaker than the twin prime conjecture

In 1940, Paul Erdős proved a significant result about gaps between consecutive primes. He demonstrated the existence of a constant c < 1 such that there are infinitely many primes p where p' − p < c ln p (where p' denotes the next prime after p). This result established that there exist infinitely many pairs of consecutive primes whose gap grows more slowly than a logarithmic function. Subsequent mathematicians improved upon Erdős's work:

In 1986, Helmut Maier showed that the constant could be reduced to c < 0.25. In 2004, Daniel Goldston and Cem Yıldırım further improved the bound to c = 0.085786... In 2005, Goldston, János Pintz, and Yıldırım made a breakthrough by proving that c could be chosen to be arbitrarily small, i.e.

$\liminf _{n\to \infty }\left({\frac {p_{n+1}-p_{n}}{\log p_{n}}}\right)=0$

However, this result does not extend to slower-growing functions, such as c ln ln p, where the question remains open. Under the assumption of the Elliott–Halberstam conjecture (or a slightly weaker version), Goldston, Pintz, and Yıldırım proved the existence of infinitely many integers n where at least two numbers from the set {n, n + 2, n + 6, n + 8, n + 12, n + 18, n + 20} are prime. With stronger hypotheses, they showed that infinitely often at least two numbers from the smaller set {n, n + 2, n + 4, n + 6} are prime.

In 2013, Yitang Zhang proved that:

$\liminf _{n\to \infty }(p_{n+1}-p_{n})<N$

where N = 7 × 10^7. This represented a significant improvement over the Goldston–Graham–Pintz–Yıldırım result. Through subsequent work by the Polymath Project and James Maynard, this bound has been substantially reduced, with the current best known value being 246.

Conjectures

First Hardy–Littlewood conjecture

The first Hardy–Littlewood conjecture (named 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 ⁠$\pi _{2}(x)$⁠ denote the number of primes p ≤ x such that p + 2 is also prime. Define the twin prime constant C₂ as $$C_{2}=\prod _{\textstyle {p\;\mathrm {prime,} \atop p\geq 3}}\left(1-{\frac {1}{(p-1)^{2}}}\right)\approx 0.660161815846869573927812110014\ldots .$$ (Here the product extends over all prime numbers p ≥ 3.) Then a special case of the first Hardy-Littlewood conjecture is that $$\pi _{2}(x)\sim 2C_{2}{\frac {x}{(\ln x)^{2}}}\sim 2C_{2}\int _{2}^{x}{\mathrm {d} t \over (\ln t)^{2}}$$ in the sense that the quotient of the two expressions tends to 1 as x approaches infinity. (The second ~ is not part of the conjecture and is proven by integration by parts.)

The conjecture can be justified (but not proven) by assuming that ⁠${\tfrac {1}{\ln t}}$⁠ describes the density function of the prime distribution. This assumption, which is suggested by the prime number theorem, implies the twin prime conjecture, as shown in the formula for ⁠$\pi _{2}(x)$⁠ above.

The fully general first Hardy–Littlewood conjecture on prime k-tuples (not given here) implies that the second Hardy–Littlewood conjecture is false.

This conjecture has been extended by Dickson's conjecture.

Polignac's conjecture

Polignac's conjecture from 1849 states that for every positive even integer k, there are infinitely many consecutive prime pairs p and p′ such that p′ − p = k (i.e. there are infinitely many prime gaps of size k). The case k = 2 is the twin prime conjecture. The conjecture has not yet been proven or disproven for any specific value of k, but Zhang's result proves that it is true for at least one (currently unknown) value of k. Indeed, if such a k did not exist, then for any positive even natural number N there are at most finitely many n such that $p_{n+1}-p_{n}=m$ for all m < N and so for n large enough we have $p_{n+1}-p_{n}>N,$ which would contradict Zhang's result.

Large twin primes

Beginning in 2007, two distributed computing projects, Twin Prime Search and PrimeGrid, have produced several record-largest twin primes. As of August 2022, the current largest twin prime pair known is 2996863034895 × 2^1290000 ± 1 , with 388,342 decimal digits. It was discovered in September 2016.

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

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 (x) ·x /(log x)² then f (x) is about 1.7 for small x and decreases towards about 1.3 as x tends to infinity. The limiting value of f (x) is conjectured to equal twice the twin prime constant (OEIS: A114907) (not to be confused with Brun's constant), according to the Hardy–Littlewood conjecture.

Other elementary properties

Every third odd number is divisible by 3, and therefore no three successive odd numbers can be prime unless one of them is 3. Therefore, 5 is the only prime that is part of two twin prime pairs. The lower member of a pair is by definition a Chen prime.

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

It has been proven that the pair (m, m + 2) is a twin prime if and only if

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

For a twin prime pair of the form (6n − 1, 6n + 1) for some natural number n > 1, n must end in the digit 0, 2, 3, 5, 7, or 8 (OEIS: A002822). If n were to end in 1 or 6, 6n would end in 6, and 6n −1 would be a multiple of 5. This is not prime unless n = 1. Likewise, if n were to end in 4 or 9, 6n would end in 4, and 6n +1 would be a multiple of 5. The same rule applies modulo any prime p ≥ 5: If n ≡ ±6⁻¹ (mod p), then one of the pair will be divisible by p and will not be a twin prime pair unless 6n = p ±1. p = 5 just happens to produce particularly simple patterns in base 10.

Isolated prime

An isolated prime (also known as single prime or non-twin 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, ... OEIS: A007510.

It follows from Brun's theorem that almost all primes are isolated in the sense that the ratio of the number of isolated primes less than a given threshold n and the number of all primes less than n tends to 1 as n tends to infinity.

See also

• Cousin prime
• Prime gap
• Prime k-tuple
• Prime quadruplet
• Prime triplet
• Sexy prime

Further reading

• Sloane, Neil; Plouffe, Simon (1995). The Encyclopedia of Integer Sequences. San Diego, CA: Academic Press. ISBN 0-12-558630-2.

External links

• "Twins", Encyclopedia of Mathematics, EMS Press, 2001 [1994]
• Top-20 Twin Primes at Chris Caldwell's Prime Pages
• Xavier Gourdon, Pascal Sebah: Introduction to Twin Primes and Brun's Constant
• "Official press release" of 58711-digit twin prime record
• Weisstein, Eric W. "Twin Primes". MathWorld.
• The 20 000 first twin primes
• Polymath: Bounded gaps between primes
• Sudden Progress on Prime Number Problem Has Mathematicians Buzzing