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4.Initial and Terminal Objects



2.Dual Category
3.Mono-, Epi- and Isomorphisms
@4.Initial and Terminal Objects
5.Products and Coproducts
6.Exponentation and Cartesian Closed Categories

Definition - Initial Objects

An object $0$ in a category $\mathcal{C}$ is called initial if it has exactly one outgoing arrow from it into every object, $A$, in $\mathcal{C}$.

Definition - Terminal Objects

An object $1$ in a category $\mathcal{C}$ is called terminal if it has exactly one incoming arrow from every object, $A$, in $\mathcal{C}$.

We often label these arrows into terminal objects or out from initial objects by ! to highlight their uniqueness.

Example - Set

In $Set$ the only initial object is the empty set $\emptyset$. Each one element set, $\{x\}$, is terminal since there is a function for some arbitrary set, $S$, that maps every element to $x$.

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