Finite

A set is said to be finite if its elements can be labeled with the numbers from \(1\) to \(n\), for some natural number \(n\). A set that isn't finite is said to be infinite.

More precisely, a set \(S\) is finite when there exists \(n\in \mathbb N\) and a bijective function \(f:S\to\{<n\}\), where \(\{<n\}\) denotes the set of naturals less than \(n\). The unique natural for which this holds is called its cardinality, although this concept may be defined in greater generality. Perhaps the simplest finite set is the empty set, whose cardinality is 0. Any singleton set \(\{a\}\) is finite and has cardinality 1.

An ordinal is called finite when it's the order type of a finite well-ordered set. It can be proven that these ordinals correspond precisely to the order types of the sets \(\{<n\}\). This allows for the identification of finite ordinals with natural numbers.

Likewise, a cardinal is called finite when it's the cardinality of a finite set. Once again, finite cardinals can be identified with the natural numbers.

Properties

• Any subset of a finite set is finite. In particular, the intersection of a finite set and any other set is finite.
• The powerset of a finite set is finite.
• The union of two finite sets, and thus of finitely many finite sets, is finite.
• The sum, product, or exponentiation of two finite ordinals is finite.
• The sum, product, or exponentiation of two finite cardinals is finite.

External links

• Wikipedia Contributors. "Finite set".

• Barile, Margherita. "Finite Set" at MathWorld.