Preimage of Image of Subset under Injection equals Subset
Jump to navigation
Jump to search
Theorem
Let $f: S \to T$ be an injection.
Then:
- $\forall A \subseteq S: A = \paren {f^{-1} \circ f} \sqbrk A$
where:
- $f \sqbrk A$ denotes the image of $A$ under $f$
- $f^{-1}$ denotes the inverse of $f$
- $f^{-1} \circ f$ denotes composition of $f^{-1}$ and $f$.
Proof
Let $f$ be an injection.
From Subset of Domain is Subset of Preimage of Image, we have that:
- $\forall A \subseteq S: A \subseteq \paren {f^{-1} \circ f} \sqbrk A$
by dint of $f$ being a relation.
So what we need to do is show that:
- $\forall A \subseteq S: \paren {f^{-1} \circ f} \sqbrk A \subseteq A$
Take any $A \subseteq S$.
Let $x \in A$.
We have:
\(\ds x\) | \(\in\) | \(\ds \paren {f^{-1} \circ f} \sqbrk A\) | ||||||||||||
\(\ds \leadsto \ \ \) | \(\ds x\) | \(\in\) | \(\ds f^{-1} \sqbrk {f \sqbrk A}\) | Definition of Composition of Mappings | ||||||||||
\(\ds \leadsto \ \ \) | \(\ds \map f x\) | \(\in\) | \(\ds f \sqbrk A\) | Definition of Inverse of Mapping | ||||||||||
\(\ds \leadsto \ \ \) | \(\ds \exists y \in A: \, \) | \(\ds \map f x\) | \(=\) | \(\ds \map f y\) | Definition of Image of Subset under Mapping | |||||||||
\(\ds \leadsto \ \ \) | \(\ds x\) | \(=\) | \(\ds y\) | Definition of Injection | ||||||||||
\(\ds \leadsto \ \ \) | \(\ds x\) | \(\in\) | \(\ds A\) | as $y \in A$ |
Thus we see that:
- $\paren {f^{-1} \circ f} \sqbrk A \subseteq A$
and hence the result:
- $\forall A \subseteq S: A = \paren {f^{-1} \circ f} \sqbrk A$
$\blacksquare$
Also see
- Subset equals Preimage of Image implies Injection
- Subset equals Preimage of Image iff Mapping is Injection
Sources
- 1960: Paul R. Halmos: Naive Set Theory ... (previous) ... (next): $\S 10$: Inverses and Composites
- 1975: T.S. Blyth: Set Theory and Abstract Algebra ... (previous) ... (next): $\S 5$. Induced mappings; composition; injections; surjections; bijections: Theorem $5.8$
- 1975: Bert Mendelson: Introduction to Topology (3rd ed.) ... (previous) ... (next): Chapter $1$: Theory of Sets: $\S 6$: Functions: Exercise $1 \ \text{(c})$
- 2000: James R. Munkres: Topology (2nd ed.) ... (previous) ... (next): $1$: Set Theory and Logic: $\S 2$: Functions