8.3.3.3 The Associator
The associator of $\mathsf{Rel}$ is the natural isomorphism
\[ \alpha ^{\mathsf{Rel}}\colon {\times }\circ {\webleft ({\webleft (\times \webright )}\times {\mathsf{id}}\webright )}\mathbin {\overset {\mathord {\sim }}{\Longrightarrow }}{\times }\circ {\webleft ({\mathsf{id}}\times {\webleft (\times \webright )}\webright )}\circ {\mathbf{\alpha }^{\mathsf{Cats}}_{\mathsf{Rel},\mathsf{Rel},\mathsf{Rel}}}\mathrlap {,} \]
as in the diagram
whose component
\[ \alpha ^{\mathsf{Rel}}_{A,B,C}\colon \webleft (A\times B\webright )\times C\mathrel {\rightarrow \kern -9.5pt\mathrlap {|}\kern 6pt}A\times \webleft (B\times C\webright ) \]
at $A,B,C\in \operatorname {\mathrm{Obj}}\webleft (\mathsf{Rel}\webright )$ is the relation defined by declaring
\[ \webleft (\webleft (a,b\webright ),c\webright ) \sim _{\alpha ^{\mathsf{Rel}}_{A,B,C}} \webleft (a',\webleft (b',c'\webright )\webright ) \]
iff $a=a'$, $b=b'$, and $c=c'$.
8.3.3.4 The Left Unitor
The left unitor of $\mathsf{Rel}$ is the natural isomorphism
whose component
\[ \lambda ^{\mathsf{Rel}}_{A} \colon \mathbb {1}_{\mathsf{Rel}}\times A \mathrel {\rightarrow \kern -9.5pt\mathrlap {|}\kern 6pt}A \]
at $A$ is defined by declaring
\[ \webleft (\star ,a\webright ) \sim _{\lambda ^{\mathsf{Rel}}_{A}} b \]
iff $a=b$.
8.3.3.5 The Right Unitor
The right unitor of $\mathsf{Rel}$ is the natural isomorphism
whose component
\[ \rho ^{\mathsf{Rel}}_{A} \colon A\times \mathbb {1}_{\mathsf{Rel}} \mathrel {\rightarrow \kern -9.5pt\mathrlap {|}\kern 6pt}A \]
at $A$ is defined by declaring
\[ \webleft (a,\star \webright ) \sim _{\rho ^{\mathsf{Rel}}_{A}} b \]
iff $a=b$.
8.3.3.6 The Symmetry
The symmetry of $\mathsf{Rel}$ is the natural isomorphism
whose component
\[ \sigma ^{\mathsf{Rel}}_{A,B} \colon A\times B \to B\times A \]
at $\webleft (A,B\webright )$ is defined by declaring
\[ \webleft (a,b\webright ) \sim _{\sigma ^{\mathsf{Rel}}_{A,B}} \webleft (b',a'\webright ) \]
iff $a=a'$ and $b=b'$.
8.3.3.7 The Internal Hom
The internal Hom of $\mathsf{Rel}$ is the functor
\[ \mathrm{Rel}\colon \mathsf{Rel}^{\mathsf{op}}\times \mathsf{Rel}\to \mathsf{Rel} \]
defined
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On objects by sending $A,B\in \operatorname {\mathrm{Obj}}\webleft (\mathsf{Rel}\webright )$ to the set $\mathrm{Rel}\webleft (A,B\webright )$ of 
of .
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On morphisms by pre/post-composition defined as in Chapter 9: Constructions With Relations, .
Let $A,B,C\in \operatorname {\mathrm{Obj}}\webleft (\mathsf{Rel}\webright )$.
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Adjointness. We have adjunctions
witnessed by bijections
\begin{align*} \mathrm{Rel}\webleft (A\times B,C\webright ) & \cong \mathrm{Rel}\webleft (A,\mathrm{Rel}\webleft (B,C\webright )\webright ),\\ \mathrm{Rel}\webleft (A\times B,C\webright ) & \cong \mathrm{Rel}\webleft (B,\mathrm{Rel}\webleft (A,C\webright )\webright ), \end{align*}
natural in $A,B,C\in \operatorname {\mathrm{Obj}}\webleft (\mathsf{Rel}\webright )$.
Indeed, we have
\begin{align*} \mathrm{Rel}\webleft (A\times B,C\webright ) & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\mathsf{Sets}\webleft (A\times B\times C,\{ \mathsf{true},\mathsf{false}\} \webright )\\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\mathrm{Rel}\webleft (A,B\times C\webright )\\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\mathrm{Rel}\webleft (A,\mathrm{Rel}\webleft (B,C\webright )\webright ), \end{align*}
and similarly for the bijection $\mathrm{Rel}\webleft (A\times B,C\webright )\cong \mathrm{Rel}\webleft (B,\mathrm{Rel}\webleft (A,C\webright )\webright )$.
8.3.3.8 The Closed Symmetric Monoidal Category of Relations
The category $\mathsf{Rel}$ admits a closed symmetric monoidal category structure consisting of
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The Underlying Category. The category $\mathsf{Rel}$ of sets and relations of Definition 8.3.2.1.1.
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The Monoidal Product. The functor
\[ \times \colon \mathrm{Rel}\times \mathrm{Rel}\to \mathrm{Rel} \]
of Definition 8.3.3.1.1.
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The Internal Hom. The internal Hom functor
\[ \mathbf{Rel}\colon \mathrm{Rel}^{\mathsf{op}}\times \mathrm{Rel}\to \mathrm{Rel} \]
of Definition 8.3.3.7.1.
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The Monoidal Unit. The functor
\[ \mathbb {1}^{\mathrm{Rel}} \colon \mathsf{pt}\to \mathrm{Rel} \]
of Definition 8.3.3.2.1.
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The Associators. The natural isomorphism
\[ \alpha ^{\mathrm{Rel}} \colon {\times }\circ {\webleft ({\times }\times \operatorname {\mathrm{id}}_{\mathrm{Rel}}\webright )} \mathbin {\overset {\mathord {\sim }}{\Longrightarrow }}{\times }\circ {\webleft (\operatorname {\mathrm{id}}_{\mathrm{Rel}}\times {\times }\webright )}\circ {\mathbf{\alpha }^{\mathsf{Cats}}_{\mathrm{Rel},\mathrm{Rel},\mathrm{Rel}}} \]
of Definition 8.3.3.3.1.
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The Left Unitors. The natural isomorphism
\[ \lambda ^{\mathrm{Rel}}\colon {\times }\circ {\webleft (\mathbb {1}^{\mathrm{Rel}}\times \operatorname {\mathrm{id}}_{\mathrm{Rel}}\webright )} \mathbin {\overset {\mathord {\sim }}{\Longrightarrow }}\mathbf{\lambda }^{\mathsf{Cats}_{\mathsf{2}}}_{\mathrm{Rel}} \]
of Definition 8.3.3.4.1.
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The Right Unitors. The natural isomorphism
\[ \rho ^{\mathrm{Rel}}\colon {\times }\circ {\webleft ({\mathsf{id}}\times {\mathbb {1}^{\mathrm{Rel}}}\webright )}\mathbin {\overset {\mathord {\sim }}{\Longrightarrow }}\mathbf{\rho }^{\mathsf{Cats}_{\mathsf{2}}}_{\mathrm{Rel}} \]
of Definition 8.3.3.5.1.
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The Symmetry. The natural isomorphism
\[ \sigma ^{\mathrm{Rel}} \colon {\times } \mathbin {\overset {\mathord {\sim }}{\Longrightarrow }}{\times }\circ {\mathbf{\sigma }^{\mathsf{Cats}_{\mathsf{2}}}_{\mathrm{Rel},\mathrm{Rel}}} \]
of Definition 8.3.3.6.1.
Omitted.
Warning: This is not a Cartesian monoidal structure, as the product on $\mathsf{Rel}$ is in fact given by the disjoint union of sets; see 
