Item 2c (009Y) — The Clowder Project

6.1.4 Elementary Properties of Pointed Sets

Proposition 6.1.4.1.1 ▶ Elementary Properties of Pointed Sets

Let $\webleft (X,x_{0}\webright )$ be a pointed set.

1.
Completeness. The category $\mathsf{Sets}_{*}$ of pointed sets and morphisms between them is complete, having in particular:
1. (a)
  Products, described as in Definition 6.2.3.1.1.
2. (b)
  Pullbacks, described as in Definition 6.2.4.1.1.
3. (c)
  Equalisers, described as in Definition 6.2.5.1.1.
2.
Cocompleteness. The category $\mathsf{Sets}_{*}$ of pointed sets and morphisms between them is cocomplete, having in particular:
1. (a)
  Coproducts, described as in Definition 6.3.3.1.1.
2. (b)
  Pushouts, described as in Definition 6.3.4.1.1;

Failure To Be Cartesian Closed. The category $\mathsf{Sets}_{*}$ is not Cartesian closed.¹

Morphisms From the Monoidal Unit. We have a bijection of sets²

\[ \mathsf{Sets}_{*}\webleft (S^{0},X\webright ) \cong X, \]

natural in $\webleft (X,x_{0}\webright )\in \operatorname {\mathrm{Obj}}\webleft (\mathsf{Sets}_{*}\webright )$, internalising also to an isomorphism of pointed sets

\[ \boldsymbol {\mathsf{Sets}}_{*}\webleft (S^{0},X\webright ) \cong \webleft (X,x_{0}\webright ), \]

again natural in $\webleft (X,x_{0}\webright )\in \operatorname {\mathrm{Obj}}\webleft (\mathsf{Sets}_{*}\webright )$.

Relation to Partial Functions. We have an equivalence of categories³

\[ \mathsf{Sets}_{*}\mathrel {\smash {\overset {\scriptscriptstyle \text{eq.}}\cong }}\mathsf{Sets}^{\mathrm{part.}} \]

between the category of pointed sets and pointed functions between them and the category of sets and partial functions between them, where:

(a)
From Pointed Sets to Sets With Partial Functions. The equivalence

\[ \xi \colon \mathsf{Sets}_{*}\mathbin {\overset {\cong }{\rightarrow }}\mathsf{Sets}^{\mathrm{part.}} \]

sends:
1. (i)
  A pointed set $\webleft (X,x_{0}\webright )$ to $X$.
2. (ii)
  A pointed function
  
  \[ f\colon \webleft (X,x_{0}\webright )\to \webleft (Y,y_{0}\webright ) \]
  
  to the partial function
  
  \[ \xi _{f}\colon X\to Y \]
  
  defined on $f^{-1}\webleft (Y\setminus y_{0}\webright )$ and given by
  
  \[ \xi _{f}\webleft (x\webright )\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}f\webleft (x\webright ) \]
  
  for each $x\in f^{-1}\webleft (Y\setminus y_{0}\webright )$.
(b)
From Sets With Partial Functions to Pointed Sets. The equivalence

\[ \xi ^{-1}\colon \mathsf{Sets}^{\mathrm{part.}}\mathbin {\overset {\cong }{\rightarrow }}\mathsf{Sets}_{*} \]

sends:
1. (i)
  A set $X$ is to the pointed set $\webleft (X,\star \webright )$ with $\star $ an element that is not in $X$.
2. (ii)
  A partial function
  
  \[ f\colon X\to Y \]
  
  defined on $U\subset X$ to the pointed function
  
  \[ \xi ^{-1}_{f}\colon \webleft (X,x_{0}\webright )\to \webleft (Y,y_{0}\webright ) \]
  
  defined by
  
  \[ \xi _{f}\webleft (x\webright )\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\begin{cases} f\webleft (x\webright ) & \text{if $x\in U$,}\\ y_{0} & \text{otherwise.} \end{cases} \]
  
  for each $x\in X$.

¹The category $\mathsf{Sets}_{*}$ does admit a natural monoidal closed structure, however; see Chapter 7: Tensor Products of Pointed Sets .
²In other words, the forgetful functor
\[ {\text{忘}}\colon \mathsf{Sets}_{*}\to \mathsf{Sets} \]
defined on objects by sending a pointed set to its underlying set is corepresentable by $S^{0}$.
³Warning: This is not an isomorphism of categories, only an equivalence.

Proof of Proposition 6.1.4.1.1.

Item 1: Completeness

This follows from (the proofs) of Definition 6.2.3.1.1, Definition 6.2.4.1.1, and Definition 6.2.5.1.1 and

Item 2: Cocompleteness

This follows from (the proofs) of Definition 6.3.3.1.1, Definition 6.3.4.1.1, and Definition 6.3.5.1.1 and

Item 3: Failure To Be Cartesian Closed

See [Yuan, Is the category of pointed sets Cartesian closed?].

Item 4: Morphisms From the Monoidal Unit

Since a morphism from $S^{0}$ to a pointed set $\webleft (X,x_{0}\webright )$ sends $0\in S^{0}$ to $x_{0}$ and then can send $1\in S^{0}$ to any element of $X$, we obtain a bijection between pointed maps $S^{0}\to X$ and the elements of $X$.

The isomorphism then

\[ \boldsymbol {\mathsf{Sets}}_{*}\webleft (S^{0},X\webright ) \cong \webleft (X,x_{0}\webright ) \]

follows by noting that $\Delta _{x_{0}}\colon S^{0}\to X$, the basepoint of $\boldsymbol {\mathsf{Sets}}_{*}\webleft (S^{0},X\webright )$, corresponds to the pointed map $S^{0}\to X$ picking the element $x_{0}$ of $X$, and thus we see that the bijection between pointed maps $S^{0}\to X$ and elements of $X$ is compatible with basepoints, lifting to an isomorphism of pointed sets.

Item 5: Relation to Partial Functions

See [Brandenburg, Why are the category of pointed sets and the category of sets and partial functions ``essentially the same''?].

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