# Strictly Monotone Function is Bijective

## Theorem

Let $f$ be a real function which is defined on $I \subseteq \R$.

Let $f$ be strictly monotone on $I$.

Let the image of $f$ be $J$.

Then $f: I \to J$ is a bijection.

## Proof

From Surjection by Restriction of Codomain, $f: I \to J$ is a surjection.

$\blacksquare$