Countable refers to something that may be counted. For example, it may refer to a group of items that may be separated into individual components.
Formally we say that a set is countably infinite if and only if there exists a one-to-one correspondence (bijection) between and , the set of natural numbers. A set is countable if it is either finite or countably infinite.
The definition may also be formulated as: a set is countable is there exists an injection from to , or if there exists a surjection from to .
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