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:

  • 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.
  • 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.

Also see

  • Results about identity elements can be found here.