A type of Relation between two Sets, such that for each element $x$ belonging to the set called domain (aka input set), there is a unique element $y$ belonging to the set called co-domain (aka output set, or range). We denote this notationally as $x \mapsto y$. We typically accompany this notation, with a notation declaring what are the domain and co-domain of the function. We denote that the function $f$ is between the domain $\mathcal{X}$ and the co-domain $\mathcal{Y}$ by:

$f: \mathcal{X} \to \mathcal{Y}$

Types of function

• Injective function
• Surjective function
• Bijection or bijective function.

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