Cousin prime
In mathematics, cousin primes are prime numbers that differ by four. Compare this with twin primes, pairs of prime numbers that differ by two, and sexy primes, pairs of prime numbers that differ by six.
The cousin primes (sequences 
and in OEIS) below 1000 are:
The only prime belonging to two pairs of cousin primes is 7. One of the numbers n, n+4, n+8 will always be divisible by 3, so n = 3 is the only case where all three are primes.
As of May 2009[update] the largest known cousin prime was (p, p + 4) for
where 9001# is a primorial. It was found by Ken Davis and has 11594 digits.
The largest known cousin probable prime is
It has 29629 digits and was found by Angel, Jobling and Augustin. While the first of these numbers has been proven prime, there is no known primality test to easily determine whether the second number is prime.
It follows from the first Hardy–Littlewood conjecture that cousin primes have the same asymptotic density as twin primes. An analogue of Brun's constant for twin primes can be defined for cousin primes, called Brun's constant for cousin primes, with the initial term (3, 7) omitted, bu the convergent sum:
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