# Identity

In mathematics, identity can refer to an equality that remains true regardless of the values of any variables that appear within it, to distinguish it from an equality which is true under more particular conditions. Alternatively, in algebra, an identity or identity element of a set S with a binary operation is an element e which combined with any element s of S produces s. Yet a third meaning is that an identity is a function f from a set S to itself, such that f(x) = x for all x in S.

A common example of the first meaning is the trigonometric identity when $\displaystyle \theta$ is considered over the set of real numbers (since that is the domain of sin and cos)

$\displaystyle ( \sin \theta)^2 + ( \cos \theta)^2 = 1,\,$

which is true for all values of $\displaystyle \theta$ , as opposed to

$\displaystyle \cos \theta = 1,\,$

which is true only for a subset of the domain.

A common example of the second meaning is addition in the real numbers, where 0 is the identity. This means that for all $\displaystyle a\in\Bbb{R}$ ,

$\displaystyle 0 + a = a,\,$
$\displaystyle a + 0 = a,\,$

and

$\displaystyle 0 + 0 = 0.\,$

A common example of the third meaning is the identity permutation, which sends each element of the set { 1, 2, ..., n } to itself.