# Definition:Preimage/Relation

## Definition

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

Let $\RR^{-1} \subseteq T \times S$ be the inverse relation to $\RR$, defined as:

$\RR^{-1} = \set {\tuple {t, s}: \tuple {s, t} \in \RR}$

### Preimage of Element

Every $s \in S$ such that $\tuple {s, t} \in \RR$ is called a preimage of $t$.

In some contexts, it is not individual elements that are important, but all elements of $S$ which are of interest.

Thus the preimage of $t \in T$ is defined as:

$\map {\RR^{-1} } t := \set {s \in S: \tuple {s, t} \in \RR}$

This can also be written:

$\map {\RR^{-1} } t := \set {s \in \Img {\RR^{-1} }: \tuple {t, s} \in \RR^{-1} }$

That is, the preimage of $t$ under $\RR$ is the image of $t$ under $\RR^{-1}$.

### Preimage of Subset

Let $Y \subseteq T$.

The preimage of $Y$ under $\RR$ is defined as:

$\RR^{-1} \sqbrk Y := \set {s \in S: \exists t \in Y: \tuple {s, t} \in \RR}$

That is, the preimage of $Y$ under $\RR$ is the image of $Y$ under $\RR^{-1}$:

$\RR^{-1} \sqbrk Y := \set {s \in S: \exists t \in Y: \tuple {t, s} \in \RR^{-1} }$

If no element of $Y$ has a preimage, then $\RR^{-1} \sqbrk Y = \O$.

### Preimage of Relation

The preimage of $\mathcal R \subseteq S \times T$ is:

$\Preimg {\mathcal R} := \mathcal R^{-1} \sqbrk T = \set {s \in S: \exists t \in T: \tuple {s, t} \in \mathcal R}$

## Also known as

A preimage is also known as an inverse image.

## Also see

• Results about preimages under relations can be found here.

## Technical Note

The $\LaTeX$ code for $\Preimg {f}$ is \Preimg {f} .

When the argument is a single character, it is usual to omit the braces:

\Preimg f