6.3.1 The Initial Pointed Set

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

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

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

  • The Cone. The collection of morphisms of pointed sets

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

    defined by

    \[ \iota _{X}\webleft (\star \webright )\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}x_{0}. \]

We claim that $\webleft (\mathrm{pt},\star \webright )$ is the initial 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

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

making the diagram

commute, namely $\iota _{X}$.


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