# Definition:Asymmetric Relation

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## Definition

Let $\RR \subseteq S \times S$ be a relation in $S$.

### Definition 1

$\RR$ is **asymmetric** if and only if:

- $\tuple {x, y} \in \RR \implies \tuple {y, x} \notin \RR$

### Definition 2

$\RR$ is **asymmetric** if and only if it and its inverse are disjoint:

- $\RR \cap \RR^{-1} = \O$

## Antisymmetric and Asymmetric Relations

Note the difference between:

- An
**asymmetric relation**, in which the fact that $\tuple {x, y} \in \RR$ means that $\tuple {y, x}$ is definitely*not*in $\RR$

and:

- An
**antisymmetric relation**, in which there*may*be instances of both $\tuple {x, y} \in \RR$ and $\tuple {y, x} \in \RR$ but if there are, then it means that $x$ and $y$ have to be the same object.

## Also defined as

Some sources (possibly erroneously or carelessly) gloss over the differences between this and the definition for an antisymmetric relation, and end up using a definition for antisymmetric which comes too close to one for **asymmetric**.

An example is 1964: Steven A. Gaal: *Point Set Topology*:

- [After having discussed antireflexivity]
*... antisymmetry expresses the additional fact that at most one of the possibilities $a \mathrel \RR b$ or $b \mathrel \RR a$ can take place.*

Some sources specifically define a relation as **anti-symmetric** what has been defined on $\mathsf{Pr} \infty \mathsf{fWiki}$ as **asymmetric**.

From 1955: John L. Kelley: *General Topology*: Chapter $0$: Relations:

*... the relation $R$ is***anti-symmetric**iff it is never the case that both $x R y$ and $y R x$.

## Examples

### Parent

Asymmetric Relation/Examples/Parent

## Also see

- Results about
**asymmetric relations**can be found**here**.

## Sources

- 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next):**asymmetric relation** - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**asymmetric relation**