# Definition:Symmetric Closure

## Definition

Let $\mathcal R$ be a relation on a set $S$.

### Definition 1

The symmetric closure of $\mathcal R$ is denoted $\mathcal R^\leftrightarrow$, and is defined as the union of $\mathcal R$ with its inverse:

$\mathcal R^\leftrightarrow = \mathcal R \cup \mathcal R^{-1}$

### Definition 2

The symmetric closure of $\mathcal R$ is denoted $\mathcal R^\leftrightarrow$, and is defined as the smallest symmetric relation on $S$ which contains $\mathcal R$.

## Equivalence of Definitions

See Equivalence of Definitions of Symmetric Closure for a proof that the definitions above are equivalent.

## Note

In contrast to reflexive and transitive relations, there is no concept of symmetric reduction. A moment's thought will establish why.