# Definition:Inverse (Abstract Algebra)/Inverse

## Definition

Let $\struct {S, \circ}$ be an algebraic structure with an identity element $e_S$.

Let $x, y \in S$ be elements.

The element $y$ is an inverse of $x$ if and only if:

$y \circ x = e_S = x \circ y$

that is, if and only if $y$ is both:

a left inverse of $x$

and:

a right inverse of $x$.

## Also known as

An inverse of $x$ is also known as a two-sided inverse of $x$, symmetric element or negative of $x$.

Some sources refer to it as a reciprocal element, which terminology is borrowed from the real numbers under multiplication.

The notation used to represent an inverse of an element depends on the set and binary operation under consideration.

Various symbols are seen for a general inverse, for example $\hat x$ and $x^*$.

If $s \in S$ has an inverse, it is denoted $s^{-1}$.

If the operation concerned is commutative, then additive notation is often used:

If $s \in S$ has an inverse, it is denoted $-s$.

## Also see

• Results about inverse elements can be found here.