# Bijection

$\def\Id {\mathop{\rm Id}}$

A function (or mapping) is called bijective if it is both one-to-one and onto, i.e., if it is both injective and surjective.

In other words, a function $f : A \to B$ from a set $A$ to a set $B$ is

a bijective function or a bijection

if and only if

$f(A) = B$ and $a_1 \ne a_2$ implies $f(a_1) \ne f(a_2)$ for all $a_1, a_2 \in A$.

#### Equivalent condition

A mapping is bijective if and only if

• it has left-sided and right-sided inverses

and therefore if and only if

• there is a unique (two-sided) inverse mapping $f^{-1}$ such that $f^{-1} \circ f = \Id_A$ and $f \circ f^{-1} = \Id_B$.

#### Application

Bijections are essential for the theory of cardinal numbers:
Two sets have the same number of elements (the same cardinality), if there is a bijective mapping between them.
By the Schröder-Bernstein theorem — and not depending on the Axiom of Choice — a bijective mapping between two sets $A$ and $B$ exists if there are injective mappings both from $A$ to $B$ and from $B$ to $A$.

#### Related notions

In certain contexts, a bijective mapping of a set $A$ onto itself is called a permutation of $A$.

A bijective homomorphism is called isomorphism, and—if domain and range coincide—automorphism.

How to Cite This Entry:
Bijection. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Bijection&oldid=30987
This article was adapted from an original article by O.A. Ivanova (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article