Injection/Examples/Square Function on Natural Numbers

Example of Injection

Let $f: \N \to \N$ be the mapping defined as:

$\forall n \in \N: \map f n = n^2$

Then $f$ is an injection, but not a surjection.

Proof

Let $g: \R_{\ge 0} \to \R$ be the mapping defined as:

$\forall x \in \R_{\ge 0}: \map g x = x^2$

We have that $f$ is a restriction of $g$.

From Positive Power Function on Non-negative Reals is Strictly Increasing, $g$ is strictly increasing.

From Strictly Monotone Real Function is Bijective, it follows that $g$ is bijective.

By definition of bijection, $g$ is an injection.

It follows from Restriction of Injection is Injection that $f$ is an injection.

$\Box$

Let $p \in \N$ be a prime number.

From Square Root of Prime is Irrational:

$\nexists n \in \N: n^2 = p$

and so $f$ is not a surjection.

$\blacksquare$

Sources

• 1972: A.G. Howson: A Handbook of Terms used in Algebra and Analysis ... (previous) ... (next): $\S 2$: Sets and functions: Some special types of function