2.1.3 Binary Products of Sets

Let $A$ and $B$ be sets.

The product of $A$ and $B$1 is the product of $A$ and $B$ in $\mathsf{Sets}$ as in , .


1Further Terminology: Also called the Cartesian product of $A$ and $B$.

The product of $A$ and $B$ is the pair $\webleft (A\times B,\webleft\{ \text{pr}_{1},\text{pr}_{2}\webright\} \webright )$ consisting of:

  1. The Limit. The set $A\times B$ defined by
    \begin{align*} A\times B & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\prod _{z\in \webleft\{ A,B\webright\} }z\\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\webleft\{ f\in \mathsf{Sets}\webleft (\webleft\{ 0,1\webright\} ,A\cup B\webright )\ \middle |\ \text{we have $f\webleft (0\webright )\in A$ and $f\webleft (1\webright )\in B$}\webright\} \\ & \cong \webleft\{ \webleft\{ \webleft\{ a\webright\} ,\webleft\{ a,b\webright\} \webright\} \in \mathcal{P}\webleft (\mathcal{P}\webleft (A\cup B\webright )\webright )\ \middle |\ \text{we have $a\in A$ and $b\in B$}\webright\} \\ & \cong \webleft\{ \begin{aligned} & \text{ordered pairs $\webleft (a,b\webright )$ with}\\ & \text{$a\in A$ and $b\in B$}\end{aligned} \webright\} .\end{align*}
  2. The Cone. The maps
    \begin{align*} \text{pr}_{1} & \colon A\times B\to A,\\ \text{pr}_{2} & \colon A\times B\to B \end{align*}

    defined by

    \begin{align*} \text{pr}_{1}\webleft (a,b\webright ) & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}a,\\ \text{pr}_{2}\webleft (a,b\webright ) & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}b \end{align*}

    for each $\webleft (a,b\webright )\in A\times B$.

We claim that $A\times B$ is the categorical product of $A$ and $B$ in $\mathsf{Sets}$. Indeed, suppose we have a diagram of the form

in $\mathsf{Sets}$. Then there exists a unique map $\phi \colon P\to A\times B$ making the diagram

commute, being uniquely determined by the conditions

\begin{align*} \text{pr}_{1}\circ \phi & = p_{1},\\ \text{pr}_{2}\circ \phi & = p_{2} \end{align*}

via

\[ \phi \webleft (x\webright )=\webleft (p_{1}\webleft (x\webright ),p_{2}\webleft (x\webright )\webright ) \]

for each $x\in P$.

Let $A$, $B$, $C$, and $X$ be sets.

  1. Functoriality. The assignments $A,B,\webleft (A,B\webright )\mapsto A\times B$ define functors
    \[ \begin{array}{ccc} A\times -\colon \mkern -15mu & \mathsf{Sets} \mkern -17.5mu& {}\mathbin {\to }\mathsf{Sets},\\ -\times B\colon \mkern -15mu & \mathsf{Sets} \mkern -17.5mu& {}\mathbin {\to }\mathsf{Sets},\\ -_{1}\times -_{2}\colon \mkern -15mu & \mathsf{Sets}\times \mathsf{Sets} \mkern -17.5mu& {}\mathbin {\to }\mathsf{Sets}, \end{array} \]

    where $-_{1}\times -_{2}$ is the functor where

    • Action on Objects. For each $\webleft (A,B\webright )\in \text{Obj}\webleft (\mathsf{Sets}\times \mathsf{Sets}\webright )$, we have

      \[ \webleft [-_{1}\times -_{2}\webright ]\webleft (A,B\webright )\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}A\times B. \]

    • Action on Morphisms. For each $\webleft (A,B\webright ),\webleft (X,Y\webright )\in \text{Obj}\webleft (\mathsf{Sets}\webright )$, the action on $\textup{Hom}$-sets

      \[ \times _{\webleft (A,B\webright ),\webleft (X,Y\webright )} \colon \mathsf{Sets}\webleft (A,X\webright )\times \mathsf{Sets}\webleft (B,Y\webright )\to \mathsf{Sets}\webleft (A\times B,X\times Y\webright ) \]

      of $\times $ at $\webleft (\webleft (A,B\webright ),\webleft (X,Y\webright )\webright )$ is defined by sending $\webleft (f,g\webright )$ to the function

      \[ f\times g\colon A\times B\to X\times Y \]

      defined by

      \[ \webleft [f\times g\webright ]\webleft (a,b\webright )\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\webleft (f\webleft (a\webright ),g\webleft (b\webright )\webright ) \]

      for each $\webleft (a,b\webright )\in A\times B$.

    and where $A\times -$ and $-\times B$ are the partial functors of $-_{1}\times -_{2}$ at $A,B\in \text{Obj}\webleft (\mathsf{Sets}\webright )$.

  2. Adjointness. We have adjunctions
    witnessed by bijections
    \begin{align*} \mathsf{Sets}\webleft (A\times B,C\webright ) & \cong \mathsf{Sets}\webleft (A,\mathsf{Sets}\webleft (B,C\webright )\webright ),\\ \mathsf{Sets}\webleft (A\times B,C\webright ) & \cong \mathsf{Sets}\webleft (B,\mathsf{Sets}\webleft (A,C\webright )\webright ), \end{align*}

    natural in $A,B,C\in \text{Obj}\webleft (\mathsf{Sets}\webright )$.

  3. Associativity. We have an isomorphism of sets
    \[ \alpha ^{\mathsf{Sets}}_{A,B,C}\colon \webleft (A\times B\webright )\times C\overset {\scriptstyle \mathord {\sim }}{\dashrightarrow }A\times \webleft (B\times C\webright ), \]

    natural in $A,B,C\in \text{Obj}\webleft (\mathsf{Sets}\webright )$.

  4. Unitality. We have isomorphisms of sets
    \begin{align*} \lambda ^{\mathsf{Sets}}_{A} & \colon \text{pt}\times A \overset {\scriptstyle \mathord {\sim }}{\dashrightarrow }A,\\ \rho ^{\mathsf{Sets}}_{A} & \colon A\times \text{pt}\overset {\scriptstyle \mathord {\sim }}{\dashrightarrow }A, \end{align*}

    natural in $A\in \text{Obj}\webleft (\mathsf{Sets}\webright )$.

  5. Commutativity. We have an isomorphism of sets
    \[ \sigma ^{\mathsf{Sets}}_{A,B}\colon A\times B \overset {\scriptstyle \mathord {\sim }}{\dashrightarrow }B\times A, \]

    natural in $A,B\in \text{Obj}\webleft (\mathsf{Sets}\webright )$.

  6. Distributivity Over Coproducts. We have isomorphisms of sets
    \begin{align*} \delta ^{\mathsf{Sets}}_{\ell } & \colon A\times \webleft (B\mathchoice {\mathbin {\textstyle \coprod }}{\mathbin {\textstyle \coprod }}{\mathbin {\scriptstyle \textstyle \coprod }}{\mathbin {\scriptscriptstyle \textstyle \coprod }}C\webright ) \overset {\scriptstyle \mathord {\sim }}{\dashrightarrow }\webleft (A\times B\webright )\mathchoice {\mathbin {\textstyle \coprod }}{\mathbin {\textstyle \coprod }}{\mathbin {\scriptstyle \textstyle \coprod }}{\mathbin {\scriptscriptstyle \textstyle \coprod }}\webleft (A\times C\webright ),\\ \delta ^{\mathsf{Sets}}_{r} & \colon \webleft (A\mathchoice {\mathbin {\textstyle \coprod }}{\mathbin {\textstyle \coprod }}{\mathbin {\scriptstyle \textstyle \coprod }}{\mathbin {\scriptscriptstyle \textstyle \coprod }}B\webright )\times C \overset {\scriptstyle \mathord {\sim }}{\dashrightarrow }\webleft (A\times C\webright )\mathchoice {\mathbin {\textstyle \coprod }}{\mathbin {\textstyle \coprod }}{\mathbin {\scriptstyle \textstyle \coprod }}{\mathbin {\scriptscriptstyle \textstyle \coprod }}\webleft (B\times C\webright ), \end{align*}

    natural in $A,B,C\in \text{Obj}\webleft (\mathsf{Sets}\webright )$.

  7. Annihilation With the Empty Set. We have isomorphisms of sets
    \begin{align*} \zeta ^{\mathsf{Sets}}_{\ell } & \colon \text{Ø}\times A \overset {\scriptstyle \mathord {\sim }}{\dashrightarrow }\text{Ø},\\ \zeta ^{\mathsf{Sets}}_{r} & \colon A\times \text{Ø}\overset {\scriptstyle \mathord {\sim }}{\dashrightarrow }\text{Ø}, \end{align*}

    natural in $A\in \text{Obj}\webleft (\mathsf{Sets}\webright )$.

  8. Distributivity Over Unions. Let $X$ be a set. For each $U,V,W\in \mathcal{P}\webleft (X\webright )$, we have equalities
    \begin{align*} U\times \webleft (V\cup W\webright ) & = \webleft (U\times V\webright )\cup \webleft (U\times W\webright ),\\ \webleft (U\cup V\webright )\times W & = \webleft (U\times W\webright )\cup \webleft (V\times W\webright ) \end{align*}

    of subsets of $\mathcal{P}\webleft (X\times X\webright )$.

  9. Distributivity Over Intersections. Let $X$ be a set. For each $U,V,W\in \mathcal{P}\webleft (X\webright )$, we have equalities
    \begin{align*} U\times \webleft (V\cap W\webright ) & = \webleft (U\times V\webright )\cap \webleft (U\times W\webright ),\\ \webleft (U\cap V\webright )\times W & = \webleft (U\times W\webright )\cap \webleft (V\times W\webright ) \end{align*}

    of subsets of $\mathcal{P}\webleft (X\times X\webright )$.

  10. Distributivity Over Differences. Let $X$ be a set. For each $U,V,W\in \mathcal{P}\webleft (X\webright )$, we have equalities
    \begin{align*} U\times \webleft (V\setminus W\webright ) & = \webleft (U\times V\webright )\setminus \webleft (U\times W\webright ),\\ \webleft (U\setminus V\webright )\times W & = \webleft (U\times W\webright )\setminus \webleft (V\times W\webright ) \end{align*}

    of subsets of $\mathcal{P}\webleft (X\times X\webright )$.

  11. Distributivity Over Symmetric Differences. Let $X$ be a set. For each $U,V,W\in \mathcal{P}\webleft (X\webright )$, we have equalities
    \begin{align*} U\times \webleft (V\mathbin {\triangle }W\webright ) & = \webleft (U\times V\webright )\mathbin {\triangle }\webleft (U\times W\webright ),\\ \webleft (U\mathbin {\triangle }V\webright )\times W & = \webleft (U\times W\webright )\mathbin {\triangle }\webleft (V\times W\webright ) \end{align*}

    of subsets of $\mathcal{P}\webleft (X\times X\webright )$.

  12. Middle-Four Exchange with Respect to Intersections. The diagram

    commutes, i.e. we have

    \[ \webleft (U\times V\webright )\cap \webleft (W\times T\webright )=\webleft (U\cap V\webright )\times \webleft (W\cap T\webright ). \]

    for each $U,V,W,T\in \mathcal{P}\webleft (X\webright )$.

  13. Symmetric Monoidality. The 8-tuple $\webleft(\phantom{\mathrlap {\alpha ^{\mathsf{Sets}}}}\mathsf{Sets}\right.$, $\times $, $\text{pt}$, $\mathsf{Sets}\webleft (-_{1},-_{2}\webright )$, $\alpha ^{\mathsf{Sets}}$, $\lambda ^{\mathsf{Sets}}$, $\rho ^{\mathsf{Sets}}$, $\left.\sigma ^{\mathsf{Sets}}\webright)$ is a closed symmetric monoidal category.
  14. Symmetric Bimonoidality. The 18-tuple
    \[ \begin{aligned} & \webleft(\mathsf{Sets},\mathchoice {\mathbin {\textstyle \coprod }}{\mathbin {\textstyle \coprod }}{\mathbin {\scriptstyle \textstyle \coprod }}{\mathbin {\scriptscriptstyle \textstyle \coprod }},\times ,\text{Ø},\text{pt},\mathsf{Sets}\webleft (-_{1},-_{2}\webright ),\alpha ^{\mathsf{Sets}},\lambda ^{\mathsf{Sets}},\rho ^{\mathsf{Sets}},\sigma ^{\mathsf{Sets}},\right.\\ & \left.\alpha ^{\mathsf{Sets},\mathchoice {\mathbin {\textstyle \coprod }}{\mathbin {\textstyle \coprod }}{\mathbin {\scriptstyle \textstyle \coprod }}{\mathbin {\scriptscriptstyle \textstyle \coprod }}},\lambda ^{\mathsf{Sets},\mathchoice {\mathbin {\textstyle \coprod }}{\mathbin {\textstyle \coprod }}{\mathbin {\scriptstyle \textstyle \coprod }}{\mathbin {\scriptscriptstyle \textstyle \coprod }}},\rho ^{\mathsf{Sets},\mathchoice {\mathbin {\textstyle \coprod }}{\mathbin {\textstyle \coprod }}{\mathbin {\scriptstyle \textstyle \coprod }}{\mathbin {\scriptscriptstyle \textstyle \coprod }}},\sigma ^{\mathsf{Sets},\mathchoice {\mathbin {\textstyle \coprod }}{\mathbin {\textstyle \coprod }}{\mathbin {\scriptstyle \textstyle \coprod }}{\mathbin {\scriptscriptstyle \textstyle \coprod }}},\delta ^{\mathsf{Sets}}_{\ell },\delta ^{\mathsf{Sets}}_{r},\zeta ^{\mathsf{Sets}}_{\ell },\zeta ^{\mathsf{Sets}}_{r}\webright),\end{aligned} \]

    is a symmetric closed bimonoidal category, where $\alpha ^{\mathsf{Sets},\mathchoice {\mathbin {\textstyle \coprod }}{\mathbin {\textstyle \coprod }}{\mathbin {\scriptstyle \textstyle \coprod }}{\mathbin {\scriptscriptstyle \textstyle \coprod }}}$, $\lambda ^{\mathsf{Sets},\mathchoice {\mathbin {\textstyle \coprod }}{\mathbin {\textstyle \coprod }}{\mathbin {\scriptstyle \textstyle \coprod }}{\mathbin {\scriptscriptstyle \textstyle \coprod }}}$, $\rho ^{\mathsf{Sets},\mathchoice {\mathbin {\textstyle \coprod }}{\mathbin {\textstyle \coprod }}{\mathbin {\scriptstyle \textstyle \coprod }}{\mathbin {\scriptscriptstyle \textstyle \coprod }}}$, and $\sigma ^{\mathsf{Sets},\mathchoice {\mathbin {\textstyle \coprod }}{\mathbin {\textstyle \coprod }}{\mathbin {\scriptstyle \textstyle \coprod }}{\mathbin {\scriptscriptstyle \textstyle \coprod }}}$ are the natural transformations from Item 2, Item 3, and Item 4 of Proposition 2.2.3.1.3.

Item 1: Functoriality
This follows from , of .
Item 2: Adjointness
We prove only that there’s an adjunction $-\times B\dashv \mathsf{Sets}\webleft (B,-\webright )$, witnessed by a bijection

\[ \mathsf{Sets}\webleft (A\times B,C\webright )\cong \mathsf{Sets}\webleft (A,\mathsf{Sets}\webleft (B,C\webright )\webright ), \]

natural in $B,C\in \text{Obj}\webleft (\mathsf{Sets}\webright )$, as the proof of the existence of the adjunction $A\times -\dashv \mathsf{Sets}\webleft (A,-\webright )$ follows almost exactly in the same way.

  • Map I. We define a map

    \[ \Phi _{B,C}\colon \mathsf{Sets}\webleft (A\times B,C\webright )\to \mathsf{Sets}\webleft (A,\mathsf{Sets}\webleft (B,C\webright )\webright ), \]

    by sending a function

    \[ \xi \colon A\times B\to C \]

    to the function

    where we define

    \[ \xi ^{\dagger }_{a}\webleft (b\webright )\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\xi \webleft (a,b\webright ) \]

    for each $b\in B$. In terms of the $[\mspace {-3mu}[a\mapsto f\webleft (a\webright )]\mspace {-3mu}]$ notation of Chapter 1: Sets, Notation 1.1.1.1.2, we have

    \[ \xi ^{\dagger }\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}[\mspace {-3mu}[a\mapsto [\mspace {-3mu}[b\mapsto \xi \webleft (a,b\webright )]\mspace {-3mu}]]\mspace {-3mu}]. \]

  • Map II. We define a map

    \[ \Psi _{B,C}\colon \mathsf{Sets}\webleft (A,\mathsf{Sets}\webleft (B,C\webright )\webright ),\to \mathsf{Sets}\webleft (A\times B,C\webright ) \]

    given by sending a function

    to the function

    \[ \xi ^{\dagger }\colon A\times B\to C \]

    defined by

    \begin{align*} \xi ^{\dagger }\webleft (a,b\webright ) & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\mathrm{ev}_{b}\webleft (\mathrm{ev}_{a}\webleft (\xi \webright )\webright )\\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\mathrm{ev}_{b}\webleft (\xi _{a}\webright )\\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\xi _{a}\webleft (b\webright )\end{align*}

    for each $\webleft (a,b\webright )\in A\times B$.

  • Invertibility I. We claim that

    \[ \Psi _{A,B}\circ \Phi _{A,B}=\text{id}_{\mathsf{Sets}\webleft (A\times B,C\webright )}. \]

    Indeed, given a function $\xi \colon A\times B\to C$, we have

    \begin{align*} \webleft [\Psi _{A,B}\circ \Phi _{A,B}\webright ]\webleft (\xi \webright ) & = \Psi _{A,B}\webleft (\Phi _{A,B}\webleft (\xi \webright )\webright )\\ & = \Psi _{A,B}\webleft (\Phi _{A,B}\webleft ([\mspace {-3mu}[\webleft (a,b\webright )\mapsto \xi \webleft (a,b\webright )]\mspace {-3mu}]\webright )\webright )\\ & = \Psi _{A,B}\webleft ([\mspace {-3mu}[a\mapsto [\mspace {-3mu}[b\mapsto \xi \webleft (a,b\webright )]\mspace {-3mu}]]\mspace {-3mu}]\webright )\\ & = \Psi _{A,B}\webleft ([\mspace {-3mu}[a'\mapsto [\mspace {-3mu}[b'\mapsto \xi \webleft (a',b'\webright )]\mspace {-3mu}]]\mspace {-3mu}]\webright )\\ & = [\mspace {-3mu}[\webleft (a,b\webright )\mapsto \mathrm{ev}_{b}\webleft (\mathrm{ev}_{a}\webleft ([\mspace {-3mu}[a'\mapsto [\mspace {-3mu}[b'\mapsto \xi \webleft (a',b'\webright )]\mspace {-3mu}]]\mspace {-3mu}]\webright )\webright )]\mspace {-3mu}]\\ & = [\mspace {-3mu}[\webleft (a,b\webright )\mapsto \mathrm{ev}_{b}\webleft ([\mspace {-3mu}[b'\mapsto \xi \webleft (a,b'\webright )]\mspace {-3mu}]\webright )]\mspace {-3mu}]\\ & = [\mspace {-3mu}[\webleft (a,b\webright )\mapsto \xi \webleft (a,b\webright )]\mspace {-3mu}]\\ & = \xi . \end{align*}

  • Invertibility II. We claim that

    \[ \Phi _{A,B}\circ \Psi _{A,B}=\text{id}_{\mathsf{Sets}\webleft (A,\mathsf{Sets}\webleft (B,C\webright )\webright )}. \]

    Indeed, given a function

    we have

    \begin{align*} \webleft [\Phi _{A,B}\circ \Psi _{A,B}\webright ]\webleft (\xi \webright ) & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\Phi _{A,B}\webleft (\Psi _{A,B}\webleft (\xi \webright )\webright )\\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\Phi _{A,B}\webleft ([\mspace {-3mu}[\webleft (a,b\webright )\mapsto \xi _{a}\webleft (b\webright )]\mspace {-3mu}]\webright )\\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\Phi _{A,B}\webleft ([\mspace {-3mu}[\webleft (a',b'\webright )\mapsto \xi _{a'}\webleft (b'\webright )]\mspace {-3mu}]\webright )\\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}[\mspace {-3mu}[a\mapsto [\mspace {-3mu}[b\mapsto \mathrm{ev}_{\webleft (a,b\webright )}\webleft ([\mspace {-3mu}[\webleft (a',b'\webright )\mapsto \xi _{a'}\webleft (b'\webright )]\mspace {-3mu}]\webright )]\mspace {-3mu}]]\mspace {-3mu}]\\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}[\mspace {-3mu}[a\mapsto [\mspace {-3mu}[b\mapsto \xi _{a}\webleft (b\webright )]\mspace {-3mu}]]\mspace {-3mu}]\\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}[\mspace {-3mu}[a\mapsto \xi _{a}]\mspace {-3mu}]\\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\xi .\end{align*}

  • Naturality for $\Phi $, Part I. We need to show that, given a function $g\colon B\to B'$, the diagram

    commutes. Indeed, given a function

    \[ \xi \colon A\times B'\to C, \]

    we have

    \begin{align*} \webleft [\Phi _{B,C}\circ \webleft (\text{id}_{A}\times g^{*}\webright )\webright ]\webleft (\xi \webright ) & = \Phi _{B,C}\webleft (\webleft [\text{id}_{A}\times g^{*}\webright ]\webleft (\xi \webright )\webright )\\ & = \Phi _{B,C}\webleft (\xi \webleft (-_{1},g\webleft (-_{2}\webright )\webright )\webright )\\ & = \webleft [\xi \webleft (-_{1},g\webleft (-_{2}\webright )\webright )\webright ]^{\dagger }\\ & = \xi ^{\dagger }_{-_{1}}\webleft (g\webleft (-_{2}\webright )\webright )\\ & = \webleft (g^{*}\webright )_{*}\webleft (\xi ^{\dagger }\webright )\\ & = \webleft (g^{*}\webright )_{*}\webleft (\Phi _{B',C}\webleft (\xi \webright )\webright )\\ & = \webleft [\webleft (g^{*}\webright )_{*}\circ \Phi _{B',C}\webright ]\webleft (\xi \webright ). \end{align*}

    Alternatively, using the $[\mspace {-3mu}[a\mapsto f\webleft (a\webright )]\mspace {-3mu}]$ notation of Chapter 1: Sets, Notation 1.1.1.1.2, we have

    \begin{align*} \webleft [\Phi _{B,C}\circ \webleft (\text{id}_{A}\times g^{*}\webright )\webright ]\webleft (\xi \webright ) & = \Phi _{B,C}\webleft (\webleft [\text{id}_{A}\times g^{*}\webright ]\webleft (\xi \webright )\webright )\\ & = \Phi _{B,C}\webleft (\webleft [\text{id}_{A}\times g^{*}\webright ]\webleft ([\mspace {-3mu}[\webleft (a,b'\webright )\mapsto \xi \webleft (a,b'\webright )]\mspace {-3mu}]\webright )\webright )\\ & = \Phi _{B,C}\webleft ([\mspace {-3mu}[\webleft (a,b\webright )\mapsto \xi \webleft (a,g\webleft (b\webright )\webright )]\mspace {-3mu}]\webright )\\ & = [\mspace {-3mu}[a\mapsto [\mspace {-3mu}[b\mapsto \xi \webleft (a,g\webleft (b\webright )\webright )]\mspace {-3mu}]]\mspace {-3mu}]\\ & = [\mspace {-3mu}[a\mapsto g^{*}\webleft ([\mspace {-3mu}[b'\mapsto \xi \webleft (a,b'\webright )]\mspace {-3mu}]\webright )]\mspace {-3mu}]\\ & = \webleft (g^{*}\webright )_{*}\webleft ([\mspace {-3mu}[a\mapsto [\mspace {-3mu}[b'\mapsto \xi \webleft (a,b'\webright )]\mspace {-3mu}]]\mspace {-3mu}]\webright )\\ & = \webleft (g^{*}\webright )_{*}\webleft (\Phi _{B',C}\webleft ([\mspace {-3mu}[\webleft (a,b'\webright )\mapsto \xi \webleft (a,b'\webright )]\mspace {-3mu}]\webright )\webright )\\ & = \webleft (g^{*}\webright )_{*}\webleft (\Phi _{B',C}\webleft (\xi \webright )\webright )\\ & = \webleft [\webleft (g^{*}\webright )_{*}\circ \Phi _{B',C}\webright ]\webleft (\xi \webright ). \end{align*}

  • Naturality for $\Phi $, Part II. We need to show that, given a function $h\colon C\to C'$, the diagram

    commutes. Indeed, given a function

    \[ \xi \colon A\times B\to C, \]

    we have

    \begin{align*} \webleft [\Phi _{B,C}\circ h_{*}\webright ]\webleft (\xi \webright ) & = \Phi _{B,C}\webleft (h_{*}\webleft (\xi \webright )\webright )\\ & = \Phi _{B,C}\webleft (h_{*}\webleft ([\mspace {-3mu}[\webleft (a,b\webright )\mapsto \xi \webleft (a,b\webright )]\mspace {-3mu}]\webright )\webright )\\ & = \Phi _{B,C}\webleft ([\mspace {-3mu}[\webleft (a,b\webright )\mapsto h\webleft (\xi \webleft (a,b\webright )\webright )]\mspace {-3mu}]\webright )\\ & = [\mspace {-3mu}[a\mapsto [\mspace {-3mu}[b\mapsto h\webleft (\xi \webleft (a,b\webright )\webright )]\mspace {-3mu}]]\mspace {-3mu}]\\ & = [\mspace {-3mu}[a\mapsto h_{*}\webleft ([\mspace {-3mu}[b\mapsto \xi \webleft (a,b\webright )]\mspace {-3mu}]]\mspace {-3mu}]\webright )\\ & = \webleft (h_{*}\webright )_{*}\webleft ([\mspace {-3mu}[a\mapsto [\mspace {-3mu}[b\mapsto \xi \webleft (a,b\webright )]\mspace {-3mu}]]\mspace {-3mu}]\webright )\\ & = \webleft (h_{*}\webright )_{*}\webleft (\Phi _{B,C}\webleft ([\mspace {-3mu}[\webleft (a,b\webright )\mapsto \xi \webleft (a,b\webright )]\mspace {-3mu}]\webright )\webright )\\ & = \webleft (h_{*}\webright )_{*}\webleft (\Phi _{B,C}\webleft (\xi \webright )\webright )\\ & = \webleft [\webleft (h_{*}\webright )_{*}\circ \Phi _{B,C}\webright ]\webleft (\xi \webright ). \end{align*}

  • Naturality for $\Psi $. Since $\Phi $ is natural in each argument and $\Phi $ is a componentwise inverse to $\Psi $ in each argument, it follows from Chapter 9: Preorders, Item 2 of Proposition 9.9.7.1.2 that $\Psi $ is also natural in each argument.
Item 3: Associativity
This is proved in the proof of Chapter 3: Monoidal Structures on the Category of Sets, Definition 3.1.4.1.1.
Item 4: Unitality
This is proved in the proof of Chapter 3: Monoidal Structures on the Category of Sets, Definition 3.1.5.1.1 and Definition 3.1.6.1.1.
Item 5: Commutativity
This is proved in the proof of Chapter 3: Monoidal Structures on the Category of Sets, Definition 3.1.7.1.1.
Item 6: Distributivity Over Coproducts
This is proved in the proof of Chapter 3: Monoidal Structures on the Category of Sets, Definition 3.3.1.1.1 and Definition 3.3.2.1.1.
Item 7: Annihilation With the Empty Set
This is proved in the proof of Chapter 3: Monoidal Structures on the Category of Sets, Definition 3.3.3.1.1 and Definition 3.3.4.1.1.
Item 8: Distributivity Over Unions
See [Proof Wiki, Cartesian Product Distributes Over Union].
Item 9: Distributivity Over Intersections
See Corollary 1 of [Proof Wiki, Cartesian Product of Intersections].
Item 10: Distributivity Over Differences
See [Proof Wiki, Cartesian Product Distributes Over Set Difference].
Item 11: Distributivity Over Symmetric Differences
See [Proof Wiki, Cartesian Product Distributes Over Symmetric Difference].
Item 12: Middle-Four Exchange With Respect to Intersections
See Corollary 1 of [Proof Wiki, Cartesian Product of Intersections].
Item 13: Symmetric Monoidality
This is a repetition of Chapter 3: Monoidal Structures on the Category of Sets, Proposition 3.1.9.1.1, and is proved there.
Item 14: Symmetric Bimonoidality
This is a repetition of Chapter 3: Monoidal Structures on the Category of Sets, Proposition 3.3.5.1.1, and is proved there.

As shown in Item 1 of Proposition 2.1.3.1.3, the Cartesian product of sets defines a functor

\[ -_{1}\times -_{2}\colon \mathsf{Sets}\times \mathsf{Sets}\to \mathsf{Sets}. \]

This functor is the $\webleft (k,\ell \webright )=\webleft (-1,-1\webright )$ case of a family of functors

\[ \otimes _{k,\ell }\colon \mathsf{Mon}_{\mathbb {E}_{k}}\webleft (\mathsf{Sets}\webright )\times \mathsf{Mon}_{\mathbb {E}_{\ell }}\webleft (\mathsf{Sets}\webright )\to \mathsf{Mon}_{\mathbb {E}_{k+\ell }}\webleft (\mathsf{Sets}\webright ) \]

of tensor products of $\mathbb {E}_{k}$-monoid objects on $\mathsf{Sets}$ with $\mathbb {E}_{\ell }$-monoid objects on $\mathsf{Sets}$; see .


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