# Cardinality of sets

#### Equal cardinality

We say that two sets \( X \) and \( Y \) have *equal cardinality* iff
there exists a bijection \( f : X \rightarrow Y \) from \( X \) to \( Y \).

#### Cardinality *n*

Let \( n \) be a natural number. A set \( X \) is said to have *cardinality n*
if it has equal cardinality with the set \( \{i \in \mathbb{N} : 1 \ge i \ge n \} \).
We also say that such a set has \( n \) elements.

#### Finite sets

A set is *finite* iff it has cardinality \( n \) for some natural number
\( n \); otherwise, the set is called *infinite*.

Notation: if \( X \) is a finite set, we use \( \#(X) \) to denote the cardinality of \( X \).

### Example

The set of natural numbers is infinite.

Sets with equal cardinality can have one contain the other. For example, there is a bijection between the set of natural numbers and the set of even natural numbers, \( f: X \rightarrow Y\), \( f(n) := 2n \).