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(0,1)-category

(0,1)-topos

# Contents

## Idea

Canonical extension provides an algebraic formulation of duality theory and a tool to derive representation theorems. It may be regarded as an algebraic formulation of Stone duality (see Gehrke-Vosmaer, p. 5).

Originally (Jonsson-Tarski) is was formulated for Boolean algebras with operators, but the notion was later generalized to distributive lattices and even to arbitrary posets.

## Definition

### For distributive lattices

Given a distributive lattice $L$, its canonical extension $L^\delta$ is the downset lattice of the poset of prime filters of $L$, ordered by inclusion.

This construction extends to a functor

$(-)^\delta : DistLattice \to DistLattice^+$

from the category of distributive lattices to that of completely distributive algebraic lattices. This is left adjoint to the corresponding forgetful functor, exhibiting completely distributive algebraic lattices as a reflective subcategory of the distributive lattices

$DistLattice^+ \stackrel{\overset{(-)^\delta}{\leftarrow}}{\hookrightarrow} DistLattice \,.$

### For Heyting algebras

The canonical extension $L^\delta$ of a distributive lattice $L$ is a complete and completely distributive lattice.

In particular the canonical extension is a Heyting algebra. If $L$ is itself already a Heyting algebra, then $e_L : L \to L^\delta$ preserves the Heyting implication. Also, canonical extension preserves homomorphisms of Heyting algebras. Hence it restricts to a functor

$(-)^\delta : HeytingAlgebra \to HeytingAlgebra \,.$

### For coherent categories

A distributive lattice is a cogerent (0,1)-category. One may therefore ask if there is a generalization of canonical extension to general coherent categories.

The following is considered in (Coumans).

By the discussion at coherent category and coherent hyperdoctrine, we have a reflective sub-2-category embedding

$(\mathcal{A} \dashv \mathcal{S}) : CoherentCat \hookrightarrow CoherentHyperdoctrine$

given by sending a coherent category $C$ to its self-indexing $c \mapsto C_{/c}$.

Since a coherent hyperdoctrine takes values in distributive lattices, we can apply canonical extension of distributive lattices termwise to get a functor

$(-)^\delta : CoherentHyperdoctrine \to CoherentHyperdoctrine \,.$

Define then canonical extension of coherent categories to be the 2-functor induced from this under the above reflection:

$(-)^\delta : CoherentCat \stackrel{\mathcal{S}}{\to} CoherentHyperdoctrine \stackrel{(-)^\delta}{\to} CoherentHyperdoctrine \stackrel{\mathcal{A}}{\to} CoherentCat \,.$

Under the restriction along the inclusion $DistLat \hookrightarrow CoherentCat$ this reproduces the canonical extension of distributive lattices: for $L$ a dsitributive lattice there is an equivalence

$\mathcal{A}(\mathcal{S}_L^\delta) \simeq L^\delta \,.$

(This is (Coumans, prop 12)).

### For Heyting categories

There is also a joint generalization of canonical extension for Heyting algebras (here) and for coherent categories (here).

Write $HeytingCat$ for the 2-category of Heyting categories, and $FirstOrderHyperdoctrine$ for the 2-category of first-order hyperdoctrines (which are, in particular, hyperdoctrines with values in Heyting algebras).

Then the above restricts to a reflective sub-2-category inclusion

$(\mathcal{A} \dashv \mathcal{S}) : HeyteingCat \hookrightarrow FirstOrderHyperdoctrine$

This is (Coumans, prop. 19).

And the canonical extension of coherent categories accordingly restricts to a functor on Heyting categories

$(-)^\delta : HeytingCat \to HeytingCat \,.$

Such that for $C$ a Heyting category, also the unit $C \to C^\delta$ is a morpjism of Heyting categories.

This is (Coumans, cor. 21).

The study of canonical extensions originates in the articles

• B. Jónsson, Alfred Tarski, Boolean algebras with operators, I, Amer. J. Math. 73 (1951), 891–939.

Reviews include

• Mai Gehrke, Jacob Vosmaer, A view of canonical extension (arXiv:1009.2803)
• Dion Coumans, Generalizing canonical extensions to the categorical setting, pdf, to appear in Annals of Pure and Applied Logic