# Renaming Mapping is Bijection

## Contents

## Theorem

Let $f: S \to T$ be a mapping.

Let $r: S / \mathcal R_f \to \Img f$ be the renaming mapping, defined as:

- $r: S / \mathcal R_f \to \Img f: \map r {\eqclass x {\mathcal R_f} } = \map f x$

where:

- $\mathcal R_f$ is the equivalence induced by the mapping $f$
- $S / \mathcal R_f$ is the quotient set of $S$ determined by $\mathcal R_f$
- $\eqclass x {\mathcal R_f}$ is the equivalence class of $x$ under $\mathcal R_f$.

The renaming mapping is a bijection.

## Proof

### Proof of Injectivity

To show that $r: S / \mathcal R_f \to \Img f$ is an injection:

\(\displaystyle \map r {\eqclass x {\mathcal R_f} }\) | \(=\) | \(\displaystyle \map r {\eqclass y {\mathcal R_f} }\) | |||||||||||

\(\displaystyle \leadsto \ \ \) | \(\displaystyle \map f x\) | \(=\) | \(\displaystyle \map f y\) | Definition of Renaming Mapping | |||||||||

\(\displaystyle \leadsto \ \ \) | \(\displaystyle x\) | \(\mathcal R_f\) | \(\displaystyle y\) | Definition of Equivalence Relation Induced by Mapping | |||||||||

\(\displaystyle \leadsto \ \ \) | \(\displaystyle \eqclass x {\mathcal R_f}\) | \(=\) | \(\displaystyle \eqclass y {\mathcal R_f}\) | Definition of Equivalence Class |

Thus $r: S / \mathcal R_f \to \Img f$ is an injection.

$\Box$

### Proof of Surjectivity

To show that $r: S / \mathcal R_f \to \Img f$ is a surjection:

Note that for all mappings $f: S \to T$, $f: S \to \Img f$ is always a surjection from Surjection by Restriction of Codomain.

Thus by definition:

- $\forall y \in \Img f: \exists x \in S: \map f x = y$

So:

\(\, \displaystyle \forall x \in S: \, \) | \(\displaystyle \exists \eqclass x {\mathcal R_f}: x\) | \(\in\) | \(\displaystyle \eqclass x {\mathcal R_f}\) | Equivalence Class is not Empty | |||||||||

\(\displaystyle \leadsto \ \ \) | \(\, \displaystyle \forall y \in \Img f: \exists \eqclass x {\mathcal R_f} \in S / \mathcal R_f: \, \) | \(\displaystyle \map f x\) | \(=\) | \(\displaystyle y\) | Definition of Equivalence Relation Induced by Mapping | ||||||||

\(\displaystyle \leadsto \ \ \) | \(\, \displaystyle \forall y \in \Img f: \exists \eqclass x {\mathcal R_f} \in S / \mathcal R_f: \, \) | \(\displaystyle \map r {\eqclass x {\mathcal R_f} }\) | \(=\) | \(\displaystyle y\) | Definition of Renaming Mapping |

Thus $r: S / \mathcal R_f \to \Img f$ is a surjection.

$\Box$

As $r: S / \mathcal R_f \to \Img f$ is both an injection and a surjection, it is by definition a bijection.

$\blacksquare$

## Different approaches

Nathan Jacobson: *Lectures in Abstract Algebra: I. Basic Concepts* considers the case where $r$ is an injection, but does not stress its bijective aspects from this particular perspective:

*This type of factorization of mappings ... is particularly useful when the set of inverse images $\map {\alpha^{-1} } {a'}$ coincides with $\overline S$; for, in this case, the mapping $\overline a$ is 1-1.*

Thus if $\overline a \overline \alpha = \overline b \overline \alpha$, then $a \alpha = b \alpha$ and $a \sim b$. Hence $\overline a = \overline b$. Thus we obtain here a factorization $\alpha = \nu \overline \alpha$ where $\overline \alpha$ is 1-1 onto $T$ and $\nu$ is the natural mapping.

Note that in the above, Jacobson uses:

- $\alpha$ for $f$
- $a'$ for the image of a representative element $a$ of $S$ under $\alpha$
- $\overline S$ for $S / \mathcal R_f$
- $\nu$ for the quotient mapping $q_{\mathcal R_f}: S \to S / \mathcal R_f$
- $\overline a$ and $\overline b$ for representative elements of $\overline S$
- $\overline \alpha$ for the renaming mapping $r$.

T.S. Blyth: *Set Theory and Abstract Algebra* takes the approach of deducing the existence of the mapping $r$, and then determining under which conditions it is either injective or surjective. From there, the surjective restriction of $r$ is taken, and $\mathcal R$ is then identified with the equivalence induced by $f$.

Hence the bijective nature of $r$ is constructed rather than deduced.

## Sources

- 1951: Nathan Jacobson:
*Lectures in Abstract Algebra: I. Basic Concepts*... (previous) ... (next): Introduction $\S 3$: Equivalence relations - 1967: George McCarty:
*Topology: An Introduction with Application to Topological Groups*... (previous) ... (next): $\text{I}$: Factoring Functions - 1975: T.S. Blyth:
*Set Theory and Abstract Algebra*... (previous) ... (next): $\S 6$. Indexed families; partitions; equivalence relations: Theorem $6.5$ - 1975: Bert Mendelson:
*Introduction to Topology*(3rd ed.) ... (previous) ... (next): Chapter $1$: Theory of Sets: $\S 7$: Relations: Exercise $2$