identity function

Given a set $S$, the **identity function** on $S$ is the function $id_S\colon S \to S$ that maps any element $x$ of $S$ to itself:

$id_S = (x \mapsto x) = \lambda x.\; x ;$

or equivalently,

$id_S(x) = x .$

The identity functions are the identity morphisms in the category Set of sets.

More generally, in any concrete category, the identity morphism of each object is given by the identity function on its underlying set.

Revised on July 20, 2012 01:37:51
by Toby Bartels
(75.88.84.217)