Definition:Identity (Abstract Algebra)

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Definition

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


Left Identity

An element $e_L \in S$ is called a left identity if and only if:

$\forall x \in S: e_L \circ x = x$


Right Identity

An element $e_R \in S$ is called a right identity if and only if:

$\forall x \in S: x \circ e_R = x$


Two-Sided Identity

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.


Also see

  • Results about identity elements can be found here.