Proprietà matematiche È il nono numero primo, after 19 and before 29. It is a prime factorial, 4! - 1. It is a safe prime number, that is to say (23-1)/2 it is still a prime number. It is a Woodall prime number. It's a Wedderburn-Etherington number. It is the second issue of Smarandache-Wellin, base concatenation 10 of the first three prime numbers,[...]