# Definition:Well-Defined/Mapping

## Contents

## Definition

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

Let $\mathcal R$ be an equivalence relation on $S$.

Let $S / \mathcal R$ be the quotient set determined by $\mathcal R$.

Let $\phi: S / \mathcal R \to T$ be a mapping such that:

- $\map \phi {\eqclass x {\mathcal R} } = \map f x$

Then $\phi: S / \mathcal R \to T$ is **well-defined** if and only if:

- $\forall \tuple {x, y} \in \mathcal R: \map f x = \map f y$

## Comment

Suppose we are given a mapping $f: S \to T$.

Suppose we have an equivalence $\mathcal R$ on $S$, and we want to define a mapping on the quotient set: $\phi: S / \mathcal R \to T$ such that $\map \phi {\eqclass \ldots {\mathcal R} } = \map f \ldots$.

That is, we want every member of the equivalence class to map to the same element.

The only way this can be done is to set $\map \phi {\eqclass x {\mathcal R} } = \map f x$.

Now, if $x, y \in S$ are in the same equivalence class with respect to $\mathcal R$, that is, in order for $\map \phi {\eqclass x {\mathcal R} }$ to make any sort of sense, we need to make sure that $\map \phi {\eqclass x {\mathcal R} } = \map \phi {\eqclass y {\mathcal R} }$, or (which comes to the same thing) $\map f x = \map f y$.

So $\map \phi {\eqclass x {\mathcal R} } = \map f x$ defines a mapping $\phi: S / \mathcal R \to T$ if and only if $\forall \tuple {x, y} \in \mathcal R: \map f x = \map f y$.

If this holds, then the mapping $\phi$ is **well-defined**.

The terminology is misleading, as $\phi$ can not be defined at all if the condition is *not* met.

What this means is: if we want to define a mapping from a quotient set to any other set, then *all* the individual elements of each equivalence class in the domain must map to the *same* element in the codomain.

Therefore, when attempting to construct or analyse such a mapping, it is necessary to check for **well-definedness**.

## Also known as

Some sources use the term **consistent** for **well-defined**.

## Sources

- 1965: J.A. Green:
*Sets and Groups*... (previous) ... (next): $\S 6.2$. Mappings of quotient sets - 1971: Allan Clark:
*Elements of Abstract Algebra*... (previous) ... (next): Chapter $1$: Equivalence Relations: $\S 19$