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

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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:

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.


Symmetry Group of Square

Consider the symmetry group of the square:

Let $\SS = ABCD$ be a square.


The various symmetry mappings 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.