Domain of Injection to Countable Set is Countable
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Theorem
Let $X$ be a set, and let $Y$ be a countable set.
Let $f: X \to Y$ be an injection.
Then $X$ is also countable.
Proof
Since $Y$ is countable, there exists an injection $g: Y \to \N$.
From Composite of Injections is Injection, $g \circ f: X \to \N$ is also an injection.
That is, $X$ is countable.
$\blacksquare$