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In mathematics, a bijection, or a bijective function, is a function f from a set X to a set Y with the property that, for every y in Y, there is exactly one x in X such that f(x) = y. It follows from this definition that no unmapped element exists in either X or Y.

Alternatively, f is bijective if it is a one-to-one correspondence between those sets; i.e., both one-to-one (injective) and onto (surjective).

For example, consider the function succ, defined from the set of integers \Z to \Z, that to each integer x associates the integer succ(x) = x + 1. For another example, consider the function sumdif that to each pair (x,y) of real numbers associates the pair sumdif(x,y) = (x + y, x − y).

A bijective function from a set to itself is also called a permutation.

The set of all bijections from X to Y is denoted as XY. (Sometimes this notation is reserved for binary relations, and bijections are denoted by XY instead.) Occasionally, the set of permutations of a single set X may be denoted X!.

Bijective functions play a fundamental role in many areas of mathematics, for instance in the definition of isomorphism (and related concepts such as homeomorphism and diffeomorphism), permutation group, projective map, and many others.


Composition and inverses

A function f is bijective if and only if its inverse relation f −1 is a function. In that case, f −1 is also a bijection.

The composition g\circ f of two bijections f:X\to Y and g:Y\to Z is a bijection. The inverse of g\circ f is (g\circ f)^{-1}=(f^{-1})\circ(g^{-1}).

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