# Polignac's conjecture

In number theory, Polignac's conjecture was made by Alphonse de Polignac in 1849 and states:

For any positive even number n, there are infinitely many prime gaps of size n. In other words: There are infinitely many cases of two consecutive prime numbers with difference n.[1]

The conjecture has not yet been proven or disproven for a given value of n. In 2013 an important breakthrough was made by Zhang Yitang who proved that there are infinitely many prime gaps of size n for some value of n < 70,000,000.[2]

For n = 2, it is the twin prime conjecture. For n = 4, it says there are infinitely many cousin primes (pp + 4). For n = 6, it says there are infinitely many sexy primes (pp + 6) with no prime between p and p + 6.

Dickson's conjecture generalizes Polignac's conjecture to cover all prime constellations; the Bateman–Horn conjecture gives conjectured asymptotic densities.

## Conjectured density

Let $\pi_n(x)$ for even n be the number of prime gaps of size n below x.

The first Hardy–Littlewood conjecture says the asymptotic density is of form

$\pi_n(x) \sim 2 C_n \frac{x}{(\ln x)^2} \sim 2 C_n \int_2^x {dt \over (\ln t)^2}$

where Cn is a function of n, and $\sim$ means that the quotient of two expressions tends to 1 as x approaches infinity.[citation needed]

C2 is the twin prime constant

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

where the product extends over all prime numbers p ≥ 3.

Cn is C2 multiplied by a number which depends on the odd prime factors q of n:

$C_n = C_2 \prod_{q|n} \frac{q-1}{q-2}.$

For example, C4 = C2 and C6 = 2C2. Twin primes have the same conjectured density as cousin primes, and half that of sexy primes.

Note that each odd prime factor q of n increases the conjectured density compared to twin primes by a factor of $\tfrac{q-1}{q-2}$. A heuristic argument follows. It relies on some unproven assumptions so the conclusion remains a conjecture. The chance of a random odd prime q dividing either a or a + 2 in a random "potential" twin prime pair is $\tfrac{2}{q}$, since q divides 1 of the q numbers from a to a + q − 1. Now assume q divides n and consider a potential prime pair (aa + n). q divides a + n if and only if q divides a, and the chance of that is $\tfrac{1}{q}$. The chance of (aa + n) being free from the factor q, divided by the chance that (a, a + 2) is free from q, then becomes $\tfrac{q-1}{q}$ divided by $\tfrac{q-2}{q}$. This equals $\tfrac{q-1}{q-2}$ which transfers to the conjectured prime density. In the case of n = 6, the argument simplifies to: If a is a random number then 3 has chance 2/3 of dividing a or a + 2, but only chance 1/3 of dividing a and a + 6, so the latter pair is conjectured twice as likely to both be prime.

## Notes

1. ^ Tattersall, J.J. (2005), Elementary number theory in nine chapters, Cambridge University Press, ISBN 978-0-521-85014-8, p. 112
2. ^ Klarreich, Erica (19 May 2013). "Unheralded Mathematician Bridges the Prime Gap". Simons Science News. Retrieved 21 May 2013.