Prime numbers are a rich and ancient source of mathematical mystery.
It has been known for over 2000 years that there are an infinite number of them. This issue's mathematical mystery concerns pairs of prime numbers called twin primes . A pair of prime numbers are twins if they differ by 2. A more general version of this question was put forward in 1850 by Alphonse de Polignac. The twin primes are used to define one of mathematics' more unusual constants.