|Number of||Conjectured infinite|
|Description||A prime which is either one more or one less than a factorial|
|First Few||2, 3, 5, 7, 11, 13, 23, 719|
Factorial Primes are prime numbers that can be expressed in the form n!±1, ie they are prime numbers that are one bigger or smaller than a factorial number. A factorial number is a number that is made by multiplying consecutive numbers, starting with 1, eg 4!=1*2*3*4. Both 11 and 13 are factorial primes because they complete the rule 4!±1.