3.1.9 The Monoidal Category of Sets and Products

The category $\mathsf{Sets}$ admits a closed symmetric monoidal category with diagonals structure consisting of:

  • The Underlying Category. The category $\mathsf{Sets}$ of pointed sets.
  • The Monoidal Product. The product functor

    \[ \times \colon \mathsf{Sets}\times \mathsf{Sets}\to \mathsf{Sets} \]

    of Chapter 2: Constructions With Sets, Item 1 of Proposition 2.1.3.1.3.

  • The Internal Hom. The internal Hom functor

    \[ \mathsf{Sets}\colon \mathsf{Sets}^{\mathsf{op}}\times \mathsf{Sets}\to \mathsf{Sets} \]

    of Chapter 2: Constructions With Sets, Item 1 of Proposition 2.3.5.1.2.

  • The Monoidal Unit. The functor

    \[ \mathbb {1}^{\mathsf{Sets}} \colon \mathsf{pt}\to \mathsf{Sets} \]

    of Definition 3.1.3.1.1.

  • The Associators. The natural isomorphism

    \[ \alpha ^{\mathsf{Sets}} \colon {\times }\circ {\webleft ({\times }\times \text{id}_{\mathsf{Sets}}\webright )} \mathbin {\overset {\mathord {\sim }}{\Longrightarrow }}{\times }\circ {\webleft (\text{id}_{\mathsf{Sets}}\times {\times }\webright )}\circ {\mathbf{\alpha }^{\mathsf{Cats}}_{\mathsf{Sets},\mathsf{Sets},\mathsf{Sets}}} \]

    of Definition 3.1.4.1.1.

  • The Left Unitors. The natural isomorphism

    \[ \lambda ^{\mathsf{Sets}}\colon {\times }\circ {\webleft (\mathbb {1}^{\mathsf{Sets}}\times \text{id}_{\mathsf{Sets}}\webright )} \mathbin {\overset {\mathord {\sim }}{\Longrightarrow }}\mathbf{\lambda }^{\mathsf{Cats}_{\mathsf{2}}}_{\mathsf{Sets}} \]

    of Definition 3.1.5.1.1.

  • The Right Unitors. The natural isomorphism

    \[ \rho ^{\mathsf{Sets}}\colon {\times }\circ {\webleft ({\mathsf{id}}\times {\mathbb {1}^{\mathsf{Sets}}}\webright )}\mathbin {\overset {\mathord {\sim }}{\Longrightarrow }}\mathbf{\rho }^{\mathsf{Cats}_{\mathsf{2}}}_{\mathsf{Sets}} \]

    of Definition 3.1.6.1.1.

  • The Symmetry. The natural isomorphism

    \[ \sigma ^{\mathsf{Sets}} \colon {\times } \mathbin {\overset {\mathord {\sim }}{\Longrightarrow }}{\times }\circ {\mathbf{\sigma }^{\mathsf{Cats}_{\mathsf{2}}}_{\mathsf{Sets},\mathsf{Sets}}} \]

    of Definition 3.1.7.1.1.

  • The Diagonals. The monoidal natural transformation

    \[ \Delta \colon \text{id}_{\mathsf{Sets}}\Longrightarrow \times \circ \Delta ^{\mathsf{Cats}_{\mathsf{2}}}_{\mathsf{Sets}} \]

    of Definition 3.1.8.1.1.

The Pentagon Identity
Let $W$, $X$, $Y$ and $Z$ be sets. We have to show that the diagram

commutes. Indeed, this diagram acts on elements as

and thus the pentagon identity is satisfied.

The Triangle Identity
Let $X$ and $Y$ be sets. We have to show that the diagram

commutes. Indeed, this diagram acts on elements as

and thus the triangle identity is satisfied.

The Left Hexagon Identity
Let $X$, $Y$, and $Z$ be sets. We have to show that the diagram

commutes. Indeed, this diagram acts on elements as

and thus the left hexagon identity is satisfied.

The Right Hexagon Identity
Let $X$, $Y$, and $Z$ be sets. We have to show that the diagram

commutes. Indeed, this diagram acts on elements as

and thus the right hexagon identity is satisfied.

Monoidal Closedness
This follows from Chapter 2: Constructions With Sets, Item 2 of Proposition 2.3.5.1.2

Existence of Monoidal Diagonals
This follows from Item 1 and Item 2 of Proposition 3.1.8.1.2.


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