Composition of Inverse Image Mappings of Mappings
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Theorem
Let $A, B, C$ be non-empty sets.
Let $f: A \to B, g: B \to C$ be mappings.
Let:
- $f^\gets: \powerset B \to \powerset A$
and
- $g^\gets: \powerset C \to \powerset B$
be the inverse image mappings of $f$ and $g$.
Then:
- $\paren {g \circ f}^\gets = f^\gets \circ g^\gets$
Proof
Let $T \subseteq C$.
We have:
\(\ds \map {\paren {f \circ g}^\gets} T\) | \(=\) | \(\ds \begin {cases} \set {x \in A: \map g {\map f x} \in T} & : \Img {g \circ f} \cap T \ne \O \\ \O & : \Img {g \circ f} \cap T = \O \end {cases}\) | |||||||||||||
and | |||||||||||||||
\(\ds \map {f^\gets \circ g^\gets} T\) | \(=\) | \(\ds \begin {cases} \set {x \in A: \map f x \in \map {g^\gets} T} & : \Img f \cap \map {g^\gets} T \ne \O \\ \O & : \Img f \cap \map {g^\gets} T = \O \end {cases}\) | |||||||||||||
\(\ds \) | \(=\) | \(\ds \begin {cases} \set {x \in A: \map g {\map f x} \in T} & : \Img f \cap \map {g^\gets} T \ne \O \\ \O & : \Img f \cap \map {g^\gets} T = \O \end {cases}\) |
It remains to be shown that:
- $\Img f \cap \map {g^\gets} T = \O \iff \Img {g \circ f} \cap T = \O$
So:
\(\ds \Img f \cap \map {g^\gets} T\) | \(=\) | \(\ds \O\) | ||||||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds \Img f\) | \(\subseteq\) | \(\ds \relcomp B {\map {g^\gets} T}\) | Empty Intersection iff Subset of Complement | ||||||||||
\(\ds \) | \(=\) | \(\ds \map {g^\gets} {\relcomp C T}\) | Complement of Preimage equals Preimage of Complement | |||||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds \Img {g \circ f}\) | \(=\) | \(\ds \map {g^\gets} {\Img f}\) | Intersection with Complement is Empty iff Subset | ||||||||||
\(\ds \) | \(\subseteq\) | \(\ds \relcomp C T\) | Subset of Preimage under Relation is Preimage of Subset: Corollary | |||||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds \Img {g \circ f} \cap T\) | \(=\) | \(\ds \O\) | Empty Intersection iff Subset of Complement |
$\blacksquare$
Sources
- 1975: T.S. Blyth: Set Theory and Abstract Algebra ... (previous) ... (next): $\S 5$. Induced mappings; composition; injections; surjections; bijections: Theorem $5.7$