Prime number

A whole number is said to be prime when its only divisors are one and itself. In more graphic terms, a prime number cannot be "broken down", in that it allows no non-trivial factorisation.

The series of prime numbers begins with 2, 3, 5, 7, 11, 13, 17, 19... For technical reasons (uniqueness of factorisation), 1 is excluded from the list.

On the other hand, 6 = 3 x 2, or 15 = 3 x 5 are not prime numbers.