Restriction of Real Square Mapping to Positive Reals is Bijection
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Theorem
Let $f: \R \to \R$ be the real square function:
- $\forall x \in \R: \map f x = x^2$
Let $g: \R_{\ge 0} \to R_{\ge 0} := f {\restriction_{\R_{\ge 0} \times R_{\ge 0} } }$ be the restriction of $f$ to the positive real numbers $\R_{\ge 0}$.
Then $g$ is a bijective restriction of $f$.
Proof
From Order is Preserved on Positive Reals by Squaring, $f$ is strictly increasing on $\R_{\ge 0}$.
By definition, a strictly increasing real function is strictly monotone.
The result follows from Strictly Monotone Real Function is Bijective.
$\blacksquare$
Sources
- 1965: J.A. Green: Sets and Groups ... (previous) ... (next): Chapter $3$. Mappings: Exercise $3$