Subsection 8.1.2 (01RD): The Reflexive Closure of a Relation

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Table of Contents
Part 2: Relations
Chapter 8: Equivalence Relations and Apartness Relations
Section 8.1: Reflexive Relations
Subsection 8.1.2: The Reflexive Closure of a Relation (cite)

8.1.2 The Reflexive Closure of a Relation

Let $R$ be a relation on $A$.

Definition 8.1.2.1.1 ▶ The Reflexive Closure of a Relation

The reflexive closure of $\mathord {\sim }_{R}$ is the relation $\smash {\mathord {\sim }^{\mathrm{refl}}_{R}}$¹ satisfying the following universal property:²

Given another reflexive relation $\mathord {\sim }_{S}$ on $A$ such that $R\subset S$, there exists an inclusion $\smash {\mathord {\sim }^{\mathrm{refl}}_{R}}\subset \mathord {\sim }_{S}$.

¹Further Notation: Also written $R^{\mathrm{refl}}$.

²Slogan: The reflexive closure of $R$ is the smallest reflexive relation containing $R$.

Construction 8.1.2.1.2 ▶ The Reflexive Closure of a Relation

Concretely, $\smash {\mathord {\sim }^{\mathrm{refl}}_{R}}$ is the free pointed object on $R$ in $\webleft (\mathbf{Rel}\webleft (A,A\webright ),\chi _{A}\webright )$¹ being given by

\begin{align*} R^{\mathrm{refl}} & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}R\mathbin {\mathchoice {\mathbin {\textstyle \coprod }}{\mathbin {\textstyle \coprod }}{\mathbin {\scriptstyle \textstyle \coprod }}{\mathbin {\scriptscriptstyle \textstyle \coprod }}^{\mathbf{Rel}\webleft (A,A\webright )}}\Delta _{A}\\ & = R\cup \Delta _{A}\\ & = \webleft\{ \webleft (a,b\webright )\in A\times A\ \middle |\ \text{we have $a\sim _{R}b$ or $a=b$}\webright\} .\end{align*}

¹Or, equivalently, the free $\mathbb {E}_{0}$-monoid on $R$ in $\webleft (\mathrm{N}_{\bullet }\webleft (\mathbf{Rel}\webleft (A,A\webright )\webright ),\chi _{A}\webright )$.

Proof of Construction 8.1.2.1.2.

Clear.

Proposition 8.1.2.1.3 ▶ Properties of the Reflexive Closure of a Relation

Let $R$ be a relation on $A$.

Adjointness. We have an adjunction
witnessed by a bijection of sets
\[ \mathbf{Rel}^{\mathsf{refl}}\webleft (R^{\mathrm{refl}},S\webright ) \cong \mathbf{Rel}\webleft (R,S\webright ), \]

natural in $R\in \text{Obj}\webleft (\mathbf{Rel}^{\mathsf{refl}}\webleft (A,A\webright )\webright )$ and $S\in \text{Obj}\webleft (\mathbf{Rel}\webleft (A,A\webright )\webright )$.
The Reflexive Closure of a Reflexive Relation. If $R$ is reflexive, then $R^{\mathrm{refl}}=R$.
Idempotency. We have
\[ \webleft (R^{\mathrm{refl}}\webright )^{\mathrm{refl}} = R^{\mathrm{refl}}. \]
Interaction With Inverses. We have
Interaction With Composition. We have