# Definition:Identity (Abstract Algebra)/Two-Sided Identity

## Definition

Let $\struct {S, \circ}$ be an algebraic structure.

An element $e \in S$ is called an **identity (element)** if and only if it is both a left identity and a right identity:

- $\forall x \in S: x \circ e = x = e \circ x$

In Identity is Unique it is established that an **identity element**, if it exists, is unique within $\struct {S, \circ}$.

Thus it is justified to refer to it as ** the identity** (of a given algebraic structure).

This **identity** is often denoted $e_S$, or $e$ if it is clearly understood what structure is being discussed.

## Also known as

Other terms which are seen that mean the same as **identity** are:

**Two-sided identity**, to reflect the fact that it is both a left identity and a right identity.**Neutral element**, which is perfectly okay, but considered slightly old-fashioned.**Unit element**, but this is not recommended as it is too easy to confuse it with a unit of a ring.**Unity**, but this is generally reserved for a ring unity or unity of field, and can also mean the number $1$.**Zero**, but it is best to reserve that term for a zero element.- The
**trivial element**, in the context of a group.

The symbols used for an **identity element** are often found to include $0$ and $1$. In the context of the general algebraic structure, these are not recommended, as this can cause confusion.

Some sources use $I$ for the **identity**.

## Examples

### Symmetry Group of Square

Consider the **symmetry group of the square**:

Let $\SS = ABCD$ be a square.

The various symmetries of $\SS$ are:

- the identity mapping $e$
- the rotations $r, r^2, r^3$ of $90^\circ, 180^\circ, 270^\circ$ around the center of $\SS$ anticlockwise respectively
- the reflections $t_x$ and $t_y$ are reflections in the $x$ and $y$ axis respectively
- the reflection $t_{AC}$ in the diagonal through vertices $A$ and $C$
- the reflection $t_{BD}$ in the diagonal through vertices $B$ and $D$.

This group is known as the **symmetry group of the square**, and can be denoted $D_4$.

The mapping $e$ which leaves $\SS$ unchanged is the **identity element**.

## Also see

- Results about
**identity elements**can be found**here**.

## Sources

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*An Introduction to Abstract Algebra*... (previous) ... (next): $\S 31$. Identity element and inverses - 1982: P.M. Cohn:
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*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**group** - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**identity** - 2017: Amol Sasane:
*A Friendly Approach to Functional Analysis*... (previous) ... (next): Chapter $\S 2.4$: Composition of continuous linear transformations