Contents

category theory

topos theory

# Contents

## Idea

The De Morganization of a topos $\mathcal{E}$ is a universal way to turn $\mathcal{E}$ into a de Morgan topos $Sh_m(\mathcal{E})$ with the use of a certain Lawvere-Tierney topology $m$, called the De Morgan topology on $\mathcal{E}$.

This can be viewed as an analogue to the Booleanization $Sh_{\neg\neg}(\mathcal{E})$ of $\mathcal{E}$ with the help of the double negation topology $\neg\neg$ .

## Definition

Let $\mathcal{E}$ be a topos. The De Morgan topology $m$ on $\mathcal{E}$ is defined as the smallest Lawvere-Tierney topology $j$ such that the canonical monomorphism $(\top,\bot): 1\coprod 1\rightarrowtail \Omega_{\neg\neg}$ is $j$-dense. The De Morganization of $\mathcal{E}$ is the associated topos $Sh_m(\mathcal{E})$ of $m$-sheaves.

### Remark

Here $\Omega_{\neg\neg}$ denotes the subobject classifier for $Sh_{\neg\neg}(\mathcal{E})$ with $\neg\neg$ the double negation topology on $\mathcal{E}$. The De Morgan topology $m$ is well-defined due to Joyal’s lemma (cf. Johnstone 1977, p.99; or Johnstone 2002, p.215). Compare its definition to this proposition about $Sh_{\neg\neg}(\mathcal{E})$ .

## Example

###### Proposition

The De Morganization of the classifying topos for the theory of fields is the classifying topos for the geometric theory of fields of finite characteristic, in which every element is algebraic over the prime field.

This is proposition 2.3 in Caramello-Johnstone (2009).

## Properties

###### Proposition

The De Morgan topology $m$ is the smallest dense topology $j$ on $\mathcal{E}$ , i.e. $j\leq \neg\neg$ , such that $Sh_j(\mathcal{E})$ is a De Morgan topos.

This appears as theorem 1 in Caramello (2009). In other words, $Sh_{m}(\mathcal{E})$ is the largest dense De Morgan subtopos of $\mathcal{E}$

###### Proposition

The De Morgan topology $m$ is the smallest topology $j$ on $\mathcal{E}$ such that all monomorphisms of the form $\neg A\vee\neg\neg A\rightarrowtail E$ for subobjects $A\rightarrowtail E$ in $\mathcal{E}$ are $j$-dense.

This appears as proposition 6.2 in Caramello (2012a).

###### Proposition

Let $\mathcal{E}$ be a topos and $m$ be the De Morgan topology on it.

• $Sh_m(\mathcal{E})=\mathcal{E}$ iff $\mathcal{E}$ is a De Morgan topos.

• For any dense topology $j$ on $\mathcal{E}$ , $Sh_m(Sh_j(\mathcal{E}))=Sh_{m\vee j}(\mathcal{E})$.

Caramello (2009), prop.1.5. In fact, in the second statement it suffices to demand that $j$ is a weakly open topology i.e. the associated sheaf functor $a_j:\mathcal{E}\to Sh_j(\mathcal{E})$ preserves the pseudo-complementation operator in the lattices of subobjects (cf. Caramello (2012, prop.4.5)).

Notice that $Sh_{m\vee j}(\mathcal{E})=Sh_{m}(\mathcal{E})\cap Sh_{ j}(\mathcal{E})$ and, accordingly, for a dense or, more generally a weakly open subtopos De Morganization simply amounts to intersection with $Sh_m(\mathcal{E})$.

## Geometric morphisms preserving De Morganizations

Notice that in analogy to $Sh_{\neg\neg}(\mathcal{E})$ and the class of skeletal geometric morphism, the universality of the De Morganization affords to define a class of m-skeletal geometric morphisms as those geometric morphisms $f:\mathcal{F}\to\mathcal{E}$ that restrict to geometric morphisms $f|_m:Sh_m(\mathcal{F})\to Sh_m(\mathcal{E})$ .

Due to a result in Johnstone (2002, p.194), this is equivalent to the preservation of $m$-dense monomorphisms by $f^\ast$.

By the above proposition, $Sh_m(Sh_{j}(\mathcal{E}))\hookrightarrow Sh_{m}(\mathcal{E})$ for $j$ dense. Accordingly, dense inclusions $Sh_{j}(\mathcal{E})\hookrightarrow\mathcal{E}$ are m-skeletal !

The characterization of Boolean toposes by skeletal morphisms carries over to m-skeletal morphisms and De Morgan toposes:

###### Proposition

A topos $\mathcal{E}$ is De Morgan iff every geometric morphism $\mathcal{F}\to\mathcal{E}$ is m-skeletal.

Proof: Assume $\mathcal{E}$ is De Morgan, then it coincides with $Sh_m(\mathcal{E})$ and $m$-sheaves of $\mathcal{F}$ necessarily have to land there.

Conversely, assume all $\mathcal{F}\to\mathcal{E}$ are m-skeletal. Then the surjective morphism $f:\gamma\mathcal{E}\to\mathcal{E}$ from the Gleason cover $\gamma\mathcal{E}$ is m-skeletal. But $\gamma\mathcal{E}$ is De Morgan and, therefore, so is $im(f)=\mathcal{E}$. $\qed$

## References

Last revised on December 16, 2017 at 06:08:17. See the history of this page for a list of all contributions to it.