canonical extension

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.

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

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

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

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

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)).

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

Revised on July 24, 2012 04:00:16
by Zoran Škoda
(190.26.78.124)