Bijection

Examples

Batting line-up of a baseball or cricket team

Consider the batting line-up of a baseball or cricket team (or any list of all the players of any sports team where every player holds a specific spot in a line-up). The set X will be the players on the team (of size nine in the case of baseball) and the set Y will be the positions in the batting order (1st, 2nd, 3rd, etc.) The "pairing" is given by which player is in what position in this order. Property (1) is satisfied since each player is somewhere in the list. Property (2) is satisfied since no player bats in two (or more) positions in the order. Property (3) says that for each position in the order, there is some player batting in that position and property (4) states that two or more players are never batting in the same position in the list.

Seats and students of a classroom

In a classroom, there are a certain number of seats. A group of students enter the room and the instructor asks them to be seated. After a quick look around the room, the instructor declares that there is a bijection between the set of students and the set of seats, where each student is paired with the seat they are sitting in. What the instructor observed in order to reach this conclusion was that:

1. Every student was in a seat (there was no one standing),
2. No student was in more than one seat,
3. Every seat had someone sitting there (there were no empty seats), and
4. No seat had more than one student in it.

The instructor was able to conclude that there were just as many seats as there were students, without having to count either set.

Fingerprints

Consider a sample of 100 distinct individuals and the corresponding set of their fingerprints. Assuming that no two individuals in that sample share the same fingerprint, the set is one-to-one (injective). Furthermore, there aren’t any fingerprints without an individual; the set is onto (surjective). Since both sets have the same cardinality and the sets are injective and surjective, then it follows there’s a bijection between the set of individuals and their fingerprints.

More mathematical examples

• For any set X, the identity function 1_X: X → X, 1_X(x) = x is bijective.
• The multiplicative inverse function gives a bijection of the unit interval (0, 1) with the semi-infinite interval (1, +∞).
• The function f: R → R, f(x) = 2x + 1 is bijective, since for each y there is a unique x = (y − 1)/2 such that f(x) = y. More generally, any linear function over the reals, f: R → R, f(x) = ax + b (where a is non-zero) is a bijection. Each real number y is obtained from (or paired with) the real number x = (y − b)/a.
• The function f: R → (−π/2, π/2), given by f(x) = arctan(x) is bijective, since each real number x is paired with exactly one angle y in the interval (−π/2, π/2) so that tan(y) = x (that is, y = arctan(x)). If the codomain (−π/2, π/2) was made larger to include an integer multiple of π/2, then this function would no longer be onto (surjective), since there is no real number which could be paired with the multiple of π/2 by this arctan function.
• The exponential function, g: R → R, g(x) = e^x, is not bijective: for instance, there is no x in R such that g(x) = −1, showing that g is not onto (surjective). However, if the codomain is restricted to the positive real numbers ${\displaystyle {\mathbb{R}}^{+}\equiv \left(0,\mathrm{\infty }\right)}$, then g would be bijective; its inverse (see below) is the natural logarithm function ln.
• The function h: R → R⁺, h(x) = x² is not bijective: for instance, h(−1) = h(1) = 1, showing that h is not one-to-one (injective). However, if the domain is restricted to ${\displaystyle {\mathbb{R}}_0^{+}\equiv \left[0,\mathrm{\infty }\right)}$, then h would be bijective; its inverse is the positive square root function.
• By Schröder–Bernstein theorem, given any two sets X and Y, and two injective functions f: X → Y and g: Y → X, there exists a bijective function h: X → Y.
• Every map from the empty set to itself is a bijection.

Inverses

A bijection f with domain X (indicated by f: X → Y in functional notation) also defines a converse relation starting in Y and going to X (by turning the arrows around). The process of "turning the arrows around" for an arbitrary function does not, in general, yield a function, but properties (3) and (4) of a bijection say that this inverse relation is a function with domain Y. Moreover, properties (1) and (2) then say that this inverse function is a surjection and an injection, that is, the inverse function exists and is also a bijection. Functions that have inverse functions are said to be invertible. A function is invertible if and only if it is a bijection.

Stated in concise mathematical notation, a function f: X → Y is bijective if and only if it satisfies the condition

for every y in Y there is a unique x in X with y = f(x).

Continuing with the baseball batting line-up example, the function that is being defined takes as input the name of one of the players and outputs the position of that player in the batting order. Since this function is a bijection, it has an inverse function which takes as input a position in the batting order and outputs the player who will be batting in that position.

Composition

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

Conversely, if the composition ${\displaystyle g\circ f}$ of two functions is bijective, it only follows that f is injective and g is surjective.

Cardinality

If X and Y are finite sets, then there exists a bijection between the two sets X and Y if and only if X and Y have the same number of elements. Indeed, in axiomatic set theory, this is taken as the definition of "same number of elements" (equinumerosity), and generalizing this definition to infinite sets leads to the concept of cardinal number, a way to distinguish the various sizes of infinite sets.

Any infinite set that has a bijection to the natural numbers is said to be countably infinite. Likewise, any infinite set that has a bijection with the integers or the rational numbers is also countably infinite, since they also have a bijection to the natural numbers. This concept is integral to determining if some functions are countable.

For example, the set of all even integers f(n)=2n is countably infinite because there’s a bijection between the even integers and the natural numbers.

Properties

• A function f: R → R is bijective if and only if its graph meets every horizontal and vertical line exactly once.
• If X is a set, then the bijective functions from X to itself, together with the operation of functional composition (${\displaystyle \circ }$), form a group, the symmetric group of X, which is denoted variously by S(X), S_X, or X! (X factorial).
• Bijections preserve cardinalities of sets: for a subset A of the domain with cardinality |A| and subset B of the codomain with cardinality |B|, one has the following equalities:

  |f(A)| = |A| and |f⁻¹(B)| = |B|.

• If X and Y are finite sets with the same cardinality, and f: X → Y, then the following are equivalent:
  1. f is a bijection.
  2. f is a surjection.
  3. f is an injection.
• For a finite set S, there is a bijection between the set of possible total orderings of the elements and the set of bijections from S to S. That is to say, the number of permutations of elements of S is the same as the number of total orderings of that set—namely, n!.

Category theory

Bijections are precisely the isomorphisms in the category Set of sets and set functions. However, the bijections are not always the isomorphisms for more complex categories. For example, in the category Grp of groups, the morphisms must be homomorphisms since they must preserve the group structure, so the isomorphisms are group isomorphisms which are bijective homomorphisms.

Generalization to partial functions

The notion of one-to-one correspondence generalizes to partial functions, where they are called partial bijections, although partial bijections are only required to be injective. The reason for this relaxation is that a (proper) partial function is already undefined for a portion of its domain; thus there is no compelling reason to constrain its inverse to be a total function, i.e. defined everywhere on its domain. The set of all partial bijections on a given base set is called the symmetric inverse semigroup.

Another way of defining the same notion is to say that a partial bijection from A to B is any relation R (which turns out to be a partial function) with the property that R is the graph of a bijection f:A′→B′, where A′ is a subset of A and B′ is a subset of B.

When the partial bijection is on the same set, it is sometimes called a one-to-one partial transformation. An example is the Möbius transformation simply defined on the complex plane, rather than its completion to the extended complex plane.