= { 1, 2, 3, 4, 5, 6, ... }

Definition: A set is defined as countable if a one-to-one and onto function (a bijection) can be formed from that set onto the set .

By this definition, the set , although infinite, are countable.

Likewise, the set of Integers is the set of all positive and negative Natural numbers with zero attached. The Integers can be described as the set of all whole numbers, and be written as , a script Z.

= { ..., -3, -2, -1, 0, 1, 2, 3, ... }

Another way to think of the Integers is:

= { 0, 1, -1, 2, -2, 3, -3, ... }

Since the Integers can be formed by this second format, the Integers, although infinite, are also countable.