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In mathematics, an identity function, also called identity map or identity transformation, is a function which does not have any effect: it always returns the same value that was used as its argument. In other words, the identity function is the function f(x) = x.
Definition Formally, if M is a set, the identity function f on M is defined to be that function with domain and codomain M which satisfies f(x) = x for all elements x in M. The identity function f on M is often denoted by idM or 1M. Algebraic property If f M → N is any function, then we have f o idM = f = idN o f (where "o" denotes function composition). In particular, idM is the identity element of the monoid of all functions from M to M. Since the identity element of a monoid is unique, 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. Examples See also | ||||||||
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