Graph of the identity function on the
real numbers
In
mathematics
Mathematics is a field of study that discovers and organizes methods, Mathematical theory, theories and theorems that are developed and Mathematical proof, proved for the needs of empirical sciences and mathematics itself. There are many ar ...
, an identity function, also called an identity relation, identity map or identity transformation, is a
function that always returns the value that was used as its
argument
An argument is a series of sentences, statements, or propositions some of which are called premises and one is the conclusion. The purpose of an argument is to give reasons for one's conclusion via justification, explanation, and/or persu ...
, unchanged. That is, when is the identity function, the
equality is true for all values of to which can be applied.
Definition
Formally, if is a
set, the identity function on is defined to be a function with as its
domain and
codomain, satisfying
In other words, the function value in the codomain is always the same as the input element in the domain . The identity function on is clearly an
injective function as well as a
surjective function (its codomain is also its
range), so it is
bijective.
The identity function on is often denoted by .
In
set theory
Set theory is the branch of mathematical logic that studies Set (mathematics), sets, which can be informally described as collections of objects. Although objects of any kind can be collected into a set, set theory – as a branch of mathema ...
, where a function is defined as a particular kind of
binary relation
In mathematics, a binary relation associates some elements of one Set (mathematics), set called the ''domain'' with some elements of another set called the ''codomain''. Precisely, a binary relation over sets X and Y is a set of ordered pairs ...
, the identity function is given by the
identity relation, or ''diagonal'' of .
Algebraic properties
If is any function, then , where "∘" denotes
function composition. In particular, is the
identity element
In mathematics, an identity element or neutral element of a binary operation is an element that leaves unchanged every element when the operation is applied. For example, 0 is an identity element of the addition of real numbers. This concept is use ...
of the
monoid of all functions from to (under function composition).
Since the identity element of a monoid is
unique, one can alternately define the identity function on to be this identity element. Such a definition generalizes to the concept of an
identity morphism in
category theory
Category theory is a general theory of mathematical structures and their relations. It was introduced by Samuel Eilenberg and Saunders Mac Lane in the middle of the 20th century in their foundational work on algebraic topology. Category theory ...
, where the
endomorphisms of need not be functions.
Properties
*The identity function is a
linear operator
In mathematics, and more specifically in linear algebra, a linear map (also called a linear mapping, linear transformation, vector space homomorphism, or in some contexts linear function) is a mapping V \to W between two vector spaces that pr ...
when applied to
vector space
In mathematics and physics, a vector space (also called a linear space) is a set (mathematics), set whose elements, often called vector (mathematics and physics), ''vectors'', can be added together and multiplied ("scaled") by numbers called sc ...
s.
*In an -
dimensional vector space
In mathematics and physics, a vector space (also called a linear space) is a set (mathematics), set whose elements, often called vector (mathematics and physics), ''vectors'', can be added together and multiplied ("scaled") by numbers called sc ...
the identity function is represented by the
identity matrix , regardless of the
basis chosen for the space.
*The identity function on the positive
integer
An integer is the number zero (0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number (−1, −2, −3, ...). The negations or additive inverses of the positive natural numbers are referred to as negative in ...
s is a
completely multiplicative function
In number theory, functions of positive integers which respect products are important and are called completely multiplicative functions or totally multiplicative functions. A weaker condition is also important, respecting only products of coprime ...
(essentially multiplication by 1), considered in
number theory
Number theory is a branch of pure mathematics devoted primarily to the study of the integers and arithmetic functions. Number theorists study prime numbers as well as the properties of mathematical objects constructed from integers (for example ...
.
*In a
metric space
In mathematics, a metric space is a Set (mathematics), set together with a notion of ''distance'' between its Element (mathematics), elements, usually called point (geometry), points. The distance is measured by a function (mathematics), functi ...
the identity function is trivially an
isometry. An object without any
symmetry
Symmetry () in everyday life refers to a sense of harmonious and beautiful proportion and balance. In mathematics, the term has a more precise definition and is usually used to refer to an object that is Invariant (mathematics), invariant und ...
has as its
symmetry group the
trivial group containing only this isometry (symmetry type ).
*In a
topological space
In mathematics, a topological space is, roughly speaking, a Geometry, geometrical space in which Closeness (mathematics), closeness is defined but cannot necessarily be measured by a numeric Distance (mathematics), distance. More specifically, a to ...
, the identity function is always
continuous.
*The identity function is
idempotent.
See also
*
Identity matrix
*
Inclusion map
*
Indicator function
In mathematics, an indicator function or a characteristic function of a subset of a set is a function that maps elements of the subset to one, and all other elements to zero. That is, if is a subset of some set , then the indicator functio ...
References
{{DEFAULTSORT:Identity Function
Functions and mappings
Elementary mathematics
Basic concepts in set theory
Types of functions
1 (number)