If for a given prime $p$ it is true that $(p - 1)! \equiv -1 \mod p^2$ , then $p$ is called a Wilson prime. Like all other primes, Wilson primes satisfy the primality condition of Wilson's theorem, but they also divide the Wilson quotient. Only three are known as of 2007, namely: 5, 13, 563 (A007540 in Sloane's OEIS, which normally requires at least four terms before accepting sequences into the table). It is not known if there are infinitely many Wilson primes exist; the fourth Wilson prime would have to be greater than $10^9$ .