6 Relations
This chapter contains some material about relations. Notably, we discuss and explore:
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The definition of relations (Section 6.1.1).
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How relations may be viewed as decategorification of profunctors (Section 6.1.2).
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The various kind of categories that relations form, namely:
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A category (Section 6.2.2).
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A monoidal category (Section 6.2.3).
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A $2$-category (Section 6.2.4).
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A double category (Section 6.2.5).
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The various categorical properties of the $2$-category of relations, including:
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The self-duality of $\mathsf{Rel}$ and $\textbf{Rel}$ (Proposition 6.3.1.1.1).
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Identifications of equivalences and isomorphisms in $\textbf{Rel}$ with bijections (Proposition 6.3.2.1.1).
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Identifications of adjunctions in $\textbf{Rel}$ with functions (Proposition 6.3.3.1.1).
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Identifications of monads in $\textbf{Rel}$ with preorders (Proposition 6.3.4.1.1).
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Identifications of comonads in $\textbf{Rel}$ with subsets (Proposition 6.3.5.1.1).
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A description of the monoids and comonoids in $\textbf{Rel}$ with respect to the Cartesian product (Remark 6.3.6.1.1).
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Characterisations of monomorphisms in $\mathsf{Rel}$ (Proposition 6.3.7.1.1).
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Characterisations of $2$-categorical notions of monomorphisms in $\textbf{Rel}$ (Proposition 6.3.8.1.1).
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Characterisations of epimorphisms in $\mathsf{Rel}$ (Proposition 6.3.9.1.1).
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Characterisations of $2$-categorical notions of epimorphisms in $\textbf{Rel}$ (Proposition 6.3.10.1.1).
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The partial co/completeness of $\mathsf{Rel}$ (Proposition 6.3.11.1.1).
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The existence or non-existence of Kan extensions and Kan lifts in $\mathsf{Rel}$ (Remark 6.3.12.1.1).
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The closedness of $\textbf{Rel}$ (Proposition 6.3.13.1.1).
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The identification of $\textbf{Rel}$ with the category of free algebras of the powerset monad on $\mathsf{Sets}$ (Proposition 6.3.14.1.1).
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A description of two notions of “skew composition” on $\mathbf{Rel}\webleft (A,B\webright )$, giving rise to left and right skew monoidal structures analogous to the left skew monoidal structure on $\mathsf{Fun}\webleft (\mathcal{C},\mathcal{D}\webright )$ appearing in the definition of a relative monad (Section 6.4 and Section 6.5).
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Section 6.1: Relations
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Subsection 6.1.1: Foundations
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Subsection 6.1.2: Relations as Decategorifications of Profunctors
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Remark 6.1.2.1.1: Relations as Decategorifications of Profunctors I
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Remark 6.1.2.1.2: Relations as Decategorifications of Profunctors II
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Subsection 6.1.3: Examples of Relations
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Subsection 6.1.4: Functional Relations
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Definition 6.1.4.1.1: Functional Relations
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Proposition 6.1.4.1.2: Properties of Functional Relations
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Subsection 6.1.5: Total Relations
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Definition 6.1.5.1.1: Total Relations
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Proposition 6.1.5.1.2: Properties of Total Relations
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Section 6.2: Categories of Relations
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Subsection 6.2.1: The Category of Relations Between Two Sets
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Definition 6.2.1.1.1: The Category of Relations Between Two Sets
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Subsection 6.2.2: The Category of Relations
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Definition 6.2.2.1.1: The Category of Relations
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Subsection 6.2.3: The Closed Symmetric Monoidal Category of Relations
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Subsubsection 6.2.3.1: The Monoidal Product
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Definition 6.2.3.1.1: The Monoidal Product of $\mathsf{Rel}$
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Subsubsection 6.2.3.2: The Monoidal Unit
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Definition 6.2.3.2.1: The Monoidal Unit of $\mathsf{Rel}$
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Subsubsection 6.2.3.3: The Associator
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Definition 6.2.3.3.1: The Associator of $\mathsf{Rel}$
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Subsubsection 6.2.3.4: The Left Unitor
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Definition 6.2.3.4.1: The Left Unitor of $\mathsf{Rel}$
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Subsubsection 6.2.3.5: The Right Unitor
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Definition 6.2.3.5.1: The Right Unitor of $\mathsf{Rel}$
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Subsubsection 6.2.3.6: The Symmetry
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Definition 6.2.3.6.1: The Symmetry of $\mathsf{Rel}$
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Subsubsection 6.2.3.7: The Internal Hom
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Definition 6.2.3.7.1: The Internal Hom of $\mathsf{Rel}$
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Proposition 6.2.3.7.2: Properties of the Internal Hom of $\mathsf{Rel}$
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Subsubsection 6.2.3.8: The Closed Symmetric Monoidal Category of Relations
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Proposition 6.2.3.8.1: The Closed Symmetric Monoidal Category of Relations
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Subsection 6.2.4: The $2$-Category of Relations
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Definition 6.2.4.1.1: The $2$-Category of Relations
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Subsection 6.2.5: The Double Category of Relations
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Subsubsection 6.2.5.1: The Double Category of Relations
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Definition 6.2.5.1.1: The Double Category of Relations
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Subsubsection 6.2.5.2: Horizontal Identities
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Definition 6.2.5.2.1: The Horizontal Identities of $\mathsf{Rel}^{\mathsf{dbl}}$
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Subsubsection 6.2.5.3: Horizontal Composition
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Definition 6.2.5.3.1: The Horizontal Composition of $\mathsf{Rel}^{\mathsf{dbl}}$
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Subsubsection 6.2.5.4: Vertical Composition of 2-Morphisms
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Definition 6.2.5.4.1: The Vertical Composition of 2-Morphisms in $\mathsf{Rel}^{\mathsf{dbl}}$
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Subsubsection 6.2.5.5: The Associators
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Definition 6.2.5.5.1: The Associators of $\mathsf{Rel}^{\mathsf{dbl}}$
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Subsubsection 6.2.5.6: The Left Unitors
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Definition 6.2.5.6.1: The Left Unitors of $\mathsf{Rel}^{\mathsf{dbl}}$
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Subsubsection 6.2.5.7: The Right Unitors
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Definition 6.2.5.7.1: The Right Unitors of $\mathsf{Rel}^{\mathsf{dbl}}$
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Section 6.3: Properties of the $2$-Category of Relations
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Subsection 6.3.1: Self-Duality
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Proposition 6.3.1.1.1: Self-Duality for the (2-)Category of Relations
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Subsection 6.3.2: Isomorphisms and Equivalences in $\textbf{Rel}$
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Proposition 6.3.2.1.1: Isomorphisms and Equivalences in $\textbf{Rel}$
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Subsection 6.3.3: Adjunctions in $\textbf{Rel}$
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Proposition 6.3.3.1.1: Adjunctions in $\textbf{Rel}$
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Subsection 6.3.4: Monads in $\textbf{Rel}$
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Proposition 6.3.4.1.1: Monads in $\textbf{Rel}$
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Subsection 6.3.5: Comonads in $\textbf{Rel}$
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Proposition 6.3.5.1.1: Comonads in $\textbf{Rel}$
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Subsection 6.3.6: Co/Monoids in $\textbf{Rel}$
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Remark 6.3.6.1.1: Co/Monoids in $\textbf{Rel}$
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Subsection 6.3.7: Monomorphisms in $\mathsf{Rel}$
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Proposition 6.3.7.1.1: Characterisations of Monomorphisms in $\mathsf{Rel}$
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Subsection 6.3.8: 2-Categorical Monomorphisms in $\textbf{Rel}$
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Proposition 6.3.8.1.1: 2-Categorical Monomorphisms in $\textbf{Rel}$
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Question 6.3.8.1.2: Better Characterisations of Representably Full Morphisms in $\textbf{Rel}$
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Subsection 6.3.9: Epimorphisms in $\mathsf{Rel}$
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Proposition 6.3.9.1.1: Characterisations of Epimorphisms in $\mathsf{Rel}$
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Subsection 6.3.10: 2-Categorical Epimorphisms in $\textbf{Rel}$
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Proposition 6.3.10.1.1: 2-Categorical Epimorphisms in $\textbf{Rel}$
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Question 6.3.10.1.2: Better Characterisations of Corepresentably Full Morphisms in $\textbf{Rel}$
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Subsection 6.3.11: Co/Limits in $\mathsf{Rel}$
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Proposition 6.3.11.1.1: Co/Limits in $\mathsf{Rel}$
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Subsection 6.3.12: Kan Extensions and Kan Lifts in $\textbf{Rel}$
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Remark 6.3.12.1.1: Kan Extensions and Kan Lifts in $\textbf{Rel}$
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Subsection 6.3.13: Closedness of $\textbf{Rel}$
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Proposition 6.3.13.1.1: Closedness of $\textbf{Rel}$
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Subsection 6.3.14: $\textbf{Rel}$ as a Category of Free Algebras
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Proposition 6.3.14.1.1: $\textbf{Rel}$ as a Category of Free Algebras
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Section 6.4: The Left Skew Monoidal Structure on $\mathbf{Rel}\webleft (A,B\webright )$
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Subsection 6.4.1: The Left Skew Monoidal Product
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Definition 6.4.1.1.1: The Left $J$-Skew Monoidal Product of $\mathbf{Rel}\webleft (A,B\webright )$
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Subsection 6.4.2: The Left Skew Monoidal Unit
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Definition 6.4.2.1.1: The Left $J$-Skew Monoidal Unit of $\mathbf{Rel}\webleft (A,B\webright )$
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Subsection 6.4.3: The Left Skew Associators
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Definition 6.4.3.1.1: The Left $J$-Skew Associator of $\mathbf{Rel}\webleft (A,B\webright )$
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Subsection 6.4.4: The Left Skew Left Unitors
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Definition 6.4.4.1.1: The Left $J$-Skew Left Unitor of $\mathbf{Rel}\webleft (A,B\webright )$
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Subsection 6.4.5: The Left Skew Right Unitors
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Definition 6.4.5.1.1: The Left $J$-Skew Right Unitor of $\mathbf{Rel}\webleft (A,B\webright )$
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Subsection 6.4.6: The Left Skew Monoidal Structure on $\mathbf{Rel}\webleft (A,B\webright )$
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Proposition 6.4.6.1.1: The Left $J$-Skew Monoidal Structure on $\mathbf{Rel}\webleft (A,B\webright )$
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Section 6.5: The Right Skew Monoidal Structure on $\mathbf{Rel}\webleft (A,B\webright )$
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Subsection 6.5.1: The Right Skew Monoidal Product
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Definition 6.5.1.1.1: The Right $J$-Skew Monoidal Product of $\mathbf{Rel}\webleft (A,B\webright )$
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Subsection 6.5.2: The Right Skew Monoidal Unit
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Definition 6.5.2.1.1: The Right $J$-Skew Monoidal Unit of $\mathbf{Rel}\webleft (A,B\webright )$
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Subsection 6.5.3: The Right Skew Associators
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Definition 6.5.3.1.1: The Right $J$-Skew Associator of $\mathbf{Rel}\webleft (A,B\webright )$
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Subsection 6.5.4: The Right Skew Left Unitors
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Definition 6.5.4.1.1: The Right $J$-Skew Left Unitor of $\mathbf{Rel}\webleft (A,B\webright )$
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Subsection 6.5.5: The Right Skew Right Unitors
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Definition 6.5.5.1.1: The Right $J$-Skew Right Unitor of $\mathbf{Rel}\webleft (A,B\webright )$
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Subsection 6.5.6: The Right Skew Monoidal Structure on $\mathbf{Rel}\webleft (A,B\webright )$
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Proposition 6.5.6.1.1: The Right $J$-Skew Monoidal Structure on $\mathbf{Rel}\webleft (A,B\webright )$