Preimage of Image of Subset under Injection equals Subset

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Let $f: S \to T$ be an injection.


$\forall A \subseteq S: A = \paren {f^{-1} \circ f} \sqbrk A$


$f \sqbrk A$ denotes the image of $A$ under $f$
$f^{-1}$ denotes the inverse of $f$
$f^{-1} \circ f$ denotes composition of $f^{-1}$ and $f$.


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:

$\forall A \subseteq S: A = \paren {f^{-1} \circ f} \sqbrk A$


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