Definition 6.3.2.1.1 (00B2): Coproducts of Families of Pointed Sets

¹Further Terminology: Also called the wedge sum of the family $\smash {\left\{ (X_{i},x^{i}_{0})\right\} _{i\in I}}$.

Concretely, the coproduct of the family $\smash {\left\{ (X_{i},x^{i}_{0})\right\} _{i\in I}}$ is the pair $\smash {((\bigvee _{i\in I}X_{i},p_{0}),\left\{ \mathrm{inj}_{i}\right\} _{i\in I})}$ consisting of:

•
The Colimit. The pointed set $(\bigvee _{i\in I}X_{i},p_{0})$ consisting of:
- •
  The Underlying Set. The set $\bigvee _{i\in I}X_{i}$ defined by
  
  \[ \bigvee _{i\in I}X_{i}\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}(\coprod _{i\in I}X_{i})/\mathord {\sim }, \]
  
  where $\mathord {\sim }$ is the equivalence relation on $\coprod _{i\in I}X_{i}$ given by declaring
  
  \[ (i,x^{i}_{0})\sim (j,x^{j}_{0}) \]
  
  for each $i,j\in I$.
- •
  The Basepoint. The element $p_{0}$ of $\bigvee _{i\in I}X_{i}$ defined by
  
  \begin{align*} p_{0} & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}[(i,x^{i}_{0})]\\ & = [(j,x^{j}_{0})] \end{align*}
  
  for any $i,j\in I$.
•
The Cocone. The collection

\[ \left\{ \mathrm{inj}_{i} \colon (X_{i},x^{i}_{0})\to (\bigvee _{i\in I}X_{i},p_{0})\right\} _{i\in I} \]

of morphism of pointed sets given by

\[ \mathrm{inj}_{i}(x)\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}(i,x) \]

for each $x\in X_{i}$ and each $i\in I$.

We claim that $\smash {(\bigvee _{i\in I}X_{i},p_{0})}$ is the categorical coproduct of $\smash {\left\{ (X_{i},x^{i}_{0})\right\} _{i\in I}}$ in $\mathsf{Sets}_{*}$. Indeed, suppose we have, for each $i\in I$, a diagram of the form

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

\[ \phi \colon (\bigvee _{i\in I}X_{i},p_{0})\to (C,*) \]

making the diagram

commute, being uniquely determined by the condition $\phi \circ \mathrm{inj}_{i}=\iota _{i}$ for each $i\in I$ via

\[ \phi ([(i,x)])=\iota _{i}(x) \]

for each $[(i,x)]\in \bigvee _{i\in I}X_{i}$, where we note that $\phi $ is indeed a morphism of pointed sets, as we have

\begin{align*} \phi (p_{0}) & = \iota _{i}([(i,x^{i}_{0})])\\ & = *, \end{align*}

as $\iota _{i}$ is a morphism of pointed sets.

Let $\left\{ (X_{i},x^{i}_{0})\right\} _{i\in I}$ be a family of pointed sets.

1.
Functoriality. The assignment $\left\{ (X_{i},x^{i}_{0})\right\} _{i\in I}\mapsto (\bigvee _{i\in I}X_{i},p_{0})$ defines a functor

\[ \bigvee _{i\in I}\colon \mathsf{Fun}(I_{\mathsf{disc}},\mathsf{Sets}_{*})\to \mathsf{Sets}_{*}. \]

Item 1: Functoriality

This follows from

The Clowder Project

6.3.2 Coproducts of Families of Pointed Sets