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Examples of Mappings which are Not Injections

Arbitrary Mapping on Sets

Let $A = \set {a, b, c, d}$.

Let $B = \set {1, 2, 3, 4, 5}$.

Let $f \subseteq {A \times B}$ be the mapping defined as:

$f = \set {\tuple {a, 1}, \tuple {b, 4}, \tuple {c, 5}, \tuple {d, 4} }$

Then $f$ is not a injection.

Square Function is Not Injective

Let $f: \R \to \R$ be the real square function:

$\forall x \in \R: \map f x = x^2$

Then $f$ is not an injection.

$\map f x = \dfrac x 2$ for $x$ Even, $0$ for $x$ Odd is Not Injective

Let $f: \Z \to Z$ be the real function defined as:

$\forall x \in \Z: \map f x = \begin{cases} \dfrac x 2 & : x \text { even} \\ 0 & : x \text { odd} \end{cases}$

Then $f$ is not an injection.