Z = set of integers {, -2, -1, 0, 1, 2, }

N = set of natural numbers {0, 1, 2, }

N = set of natural numbers {0, 1, 2, }

- For two integers d and a, d
a (d
**divides**a) if a = kd, k Z. In this case, a is a**multiple**of d, and d is a**divisor**of a (if d >= 0). Every integer divides 0.Examples: 2 8, 3 9, 2 10

- Every integer a has the
**trivial divisors**1 and a. - Nontrivial divisors are called
**factors**.Examples: 2 is a factor of 8 and 10, 3 is a factor of 9.

- An integer a > 1 with only trivial divisors is a
**prime**number; otherwise, a is a**composite**. The integers {, -2, -1, 0, 1} are neither prime nor composite. There are infinitely many prime numbers.