Identity function

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Short description: In mathematics, a function that always returns the same value that was used as its argument

thumb|[[Graph of a function|Graph of the identity function on the real numbers]]

In mathematics, 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, unchanged. That is, when f is the identity function, the equality f(x) = x is true for all values of x to which f can be applied.


Formally, if X is a set, the identity function f on X is defined to be a function with X as its domain and codomain, satisfying

f(x) = x   for all elements x in X.[1]

In other words, the function value f(x) in the codomain X is always the same as the input element x in the domain X. The identity function on X is clearly an injective function as well as a surjective function, so it is bijective.[2]

The identity function f on X is often denoted by idX.

In set theory, where a function is defined as a particular kind of binary relation, the identity function is given by the identity relation, or diagonal of X.[3]

Algebraic properties

If f : XY is any function, then we have f ∘ idX = f = idYf (where "∘" denotes function composition). In particular, idX is the identity element of the monoid of all functions from X to X (under function composition).

Since the identity element of a monoid is unique,[4] one can alternately define the identity function on M to be this identity element. Such a definition generalizes to the concept of an identity morphism in category theory, where the endomorphisms of M need not be functions.


See also


  1. Knapp, Anthony W. (2006), Basic algebra, Springer, ISBN 978-0-8176-3248-9 
  2. Mapa, Sadhan Kumar (7 April 2014). Higher Algebra Abstract and Linear (11th ed.). Sarat Book House. p. 36. ISBN 978-93-80663-24-1. 
  3. (in en) Proceedings of Symposia in Pure Mathematics. American Mathematical Society. 1974. pp. 92. ISBN 978-0-8218-1425-3.,+or+diagonal. "...then the diagonal set determined by M is the identity relation..." 
  4. Rosales, J. C.; García-Sánchez, P. A. (1999) (in en). Finitely Generated Commutative Monoids. Nova Publishers. pp. 1. ISBN 978-1-56072-670-8. "The element 0 is usually referred to as the identity element and if it exists, it is unique" 
  5. Anton, Howard (2005), Elementary Linear Algebra (Applications Version) (9th ed.), Wiley International 
  6. T. S. Shores (2007). Applied Linear Algebra and Matrix Analysis. Undergraduate Texts in Mathematics. Springer. ISBN 978-038-733-195-9. 
  7. D. Marshall; E. Odell; M. Starbird (2007). Number Theory through Inquiry. Mathematical Association of America Textbooks. Mathematical Assn of Amer. ISBN 978-0883857519. 
  8. James W. Anderson, Hyperbolic Geometry, Springer 2005, ISBN 1-85233-934-9
  9. Conover, Robert A. (2014-05-21) (in en). A First Course in Topology: An Introduction to Mathematical Thinking. Courier Corporation. pp. 65. ISBN 978-0-486-78001-6. 
  10. Conferences, University of Michigan Engineering Summer (1968) (in en). Foundations of Information Systems Engineering. "we see that an identity element of a semigroup is idempotent."