Integer Square Function is not Surjective
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Example of Mapping which is Not a Surjection
Let $f: \Z \to \Z$ be the real square function:
- $\forall x \in \Z: \map f x = x^2$
Then $f$ is not a surjection.
Proof
For $f$ to be a surjection, it would be necessary that:
- $\forall y \in \Z: \exists x \in \Z: \map f x = y$
However from Square of Real Number is Non-Negative:
- $\forall y \in \Z_{< 0}: \nexists x \in \Z: \map f x = y$
We also have that, for example, Square Root of Prime is Irrational.
So for all $p \in \Z$ such that $p$ is prime:
- $\nexists x \in \Z: \map f x = p$
Hence $f$ is not a surjection.
$\blacksquare$
Sources
- 1965: J.A. Green: Sets and Groups ... (previous) ... (next): Chapter $3$. Mappings: Exercise $2 \ \text {(i) (b)}$