6.2.1 The Terminal Pointed Set

The terminal pointed set is the terminal object of $\mathsf{Sets}_{*}$ as in Unresolved reference, Unresolved reference.

Concretely, the terminal pointed set is the pair $\smash {((\mathrm{pt},\star ),\left\{ !_{X}\right\} _{(X,x_{0})\in \operatorname {\mathrm{Obj}}(\mathsf{Sets}_{*})})}$ consisting of:

  • The Limit. The pointed set $(\mathrm{pt},\star )$.

  • The Cone. The collection of morphisms of pointed sets

    \[ \left\{ !_{X}\colon (X,x_{0})\to (\mathrm{pt},\star )\right\} _{(X,x_{0})\in \operatorname {\mathrm{Obj}}(\mathsf{Sets})} \]

    defined by

    \[ !_{X}(x)\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\star \]

    for each $x\in X$ and each $(X,x_{0})\in \operatorname {\mathrm{Obj}}(\mathsf{Sets})$.

We claim that $(\mathrm{pt},\star )$ is the terminal object of $\mathsf{Sets}_{*}$. Indeed, suppose we have a diagram of the form

in $\mathsf{Sets}_{*}$. Then there exists a unique morphism of pointed sets

\[ \phi \colon (X,x_{0})\to (\mathrm{pt},\star ) \]

making the diagram

commute, namely $!_{X}$.


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