Contents

topos theory

# Contents

## Definition

In intrinsic terms, a topos is localic if it is generated under colimits by the subobjects of its terminal object $1$.

In equivalent but extrinsic terms, a category is a localic topos if it is equivalent to the category of sheaves on a locale with respect to the topology of jointly epimorphic families (accordingly, every localic topos is a Grothendieck topos).

The frame of opens specifying the locale may indeed be taken as the poset of subobjects of $1$ (i.e., internal truth values). From the perspective of logic, localic toposes are those categories which are equivalent to the category of partial equivalence relations of the tripos given by a complete Heyting algebra (as before, the complete Heyting algebra may be taken as the poset of internal truth values).

## Properties

• A Grothendieck topos $E$ is a localic topos if and only if its unique global section geometric morphism to Set is a localic geometric morphism.

Thus, in general we regard a localic geometric morphism $E \to S$ as exhibiting E as a “localic S-topos”.

• Moreover, just as localic topoi can be identified with locales, for any base topos $S$ the 2-category of localic $S$-topoi is equivalent to the 2-category Loc$(S)$ of internal locales in $S$.

$LocTopos(S) \simeq (Topos/S)_{loc} \simeq Loc(S) \,.$

Here $LocTopos(S)$ is the 2-category whose

Then the 2-category $LocTopos$ is equivalent to the 2-category $Loc$ of locales (see C1.4.5 in the Elephant).

The 2-category $Loc$ is actually a (1,2)-category; its 2-morphism are the pointwise ordering of frame homomorphisms. Thus this equivalence implies that $LocTopos$ is also a (1,2)-category, and moreover that it is locally essentially small, in the sense that its hom-categories are essentially small. (The 2-category $Topos$ of all toposes is not locally essentially small.) Assuming sufficient separation axioms, the hom-posets of $Loc$, and hence $LocTopos$, become discrete.

## Examples

Many familiar toposes $E$, even when they are not localic, can be covered by a localic slice $E/X$ (“covered” means the unique map $X \to 1$ is an epi). For example, if $G$ is a group, then $E = Set^G$ is not itself localic, but it has a localic slice $Set^G/G \simeq Set$ that covers it. Such a topos is called an étendue (cf. Lawvere’s 1975 monograph Variable Sets Etendu and Variable Structure in Topoi).1

A significant result due to Joyal and Tierney is that for any Grothendieck topos $E$, there exists an open surjection $F \to E$ where $F$ is localic. This fact is reproduced in Mac Lane and Moerdijk’s text Sheaves in Geometry and Logic (section IX.9), where the localic cover taken is called the Diaconescu cover of $E$.

Then, using methods of descent theory, Joyal and Tierney deduce that every Grothendieck topos is equivalent to the category $B G$ of continuous discrete representations of a localic groupoid $G$. (Their result is relativized so as to hold internally over any Grothendieck topos $S$ as base.) This should be regarded as a major extrapolation of Grothendieck’s Galois theory (as in SGA 1), where it is shown that the etale topos of a field $k$ is equivalent to the category of continuous discrete representations of the fundamental pro-group $Gal(\bar{k}/k)$, where $\bar{k}$ denotes the separable closure of $k$. It was a watershed event for the penetration of localic methods in topos theory.

## The connection with propositional logic

Recall that a geometric theory $\mathbb{P}$ over a signature with no sort symbols is called propositional. Such a signature can contain at most 0-ary relation symbols but lacks variables and, accordingly, $\mathbb{P}$ admits only sequents over the empty context consisting of nested conjunctions or (infinitary) disjunctions of such relation symbols - in other words, logic boils down to propositional logic.

Propositional theories have the peculiarity that their classifying toposes always exist regardless of the availability of a natural numbers object in the base topos (cf. for details&references see at classifying topos).

###### Proposition

Localic toposes correspond exactly to classifying toposes of propositional theories.

This appears in Johnstone (2002, D3.1.14, p.897f.).

Given a locale $L$, the theory of completely prime filters $\mathbb{P}_L$ has a 0-ary relation symbol $F_x$ for each $x\in L$, thought to express the proposition that $x$ is contained in the filter $F$, and the following sequents:

• $\top\vdash F_1$ ,

• $F_x\wedge F_y\vdash F_{x\wedge y}$ for all pairs $x,y\in L$,

• $F_{\big (\bigvee_{i\in I} y_i\big )}\vdash \bigvee_{i\in I} F_{y_i}$ .

In $Set$ models of $\mathbb{P}_L$ correspond precisely to completely prime filters i.e. the multiplicatively closed subsets of $L$ containing $1$ that are inaccessible by infinite joins ($(\bigvee y_i)\in F$ implies $y_i\in F$ for some $i$). Note in particular, that the properness $0\notin F$ of a completely prime filter $F$ is implicit in the third axiom schema with $I=\emptyset$.

The relation between $L$ and $\mathbb{P}_L$ is that $Sh(L)\simeq Set[\mathbb{P}_L]$.

Conversely, given a propositional theory $\mathbb{P}$, the Lindenbaum-Tarski algebra of classes of provably equivalent formulas over $\mathbb{P}$ together with the entailment order yields a locale $L_{\mathbb{P}}$ such that $Sh(L_\mathbb{P})$ classifies $\mathbb{P}$.

### The initial and terminal toposes as classifying toposes

For illustration let us consider the empty theory $\mathbb{T}_1$ over the empty signature i.e $\mathbb{T}_1$ has no axioms. This is certainly propositional, its deductive closure consists of all tautologies using $\bot,\top,\wedge,\bigvee$. The Lindenbaum-Tarski algebra is simply $\mathbf{2}\simeq\{[\bot]\leq[\top]\}$ which corresponds to the frame of open sets of the one-point space whence $Set[\mathbb{T}_1]=Sh(\mathbf{2})=Set$. Similarly, $\mathbb{P}_{\mathbf{1}}$, the theory of completely prime filters of the one-point locale $\mathbf{1}$, is classified by $Sh(\emptyset)$ i.e. sheaves on the empty space.

Whereas $\mathbb{P}_{\mathbf{2}}$ has up to isomorphism exactly one model in every topos $\mathcal{E}$ namely the one interpreting $F_1$ as $id_{1_\mathcal{E}}$ and $F_0$ as $0_{\mathcal{E}}\hookrightarrow 1_{\mathcal{E}}$, $\mathbb{P}_{\mathbf{1}}$ has up to isomorphism exactly one model in up to isomorphism exactly one topos i.e. its model is the zero object of the (necessarily) degenerate topos.

Note that since $Set$ is the terminal Grothendieck topos, the empty theory $\mathbb{T}_1$ over the empty signature is Morita equivalent to any other geometric theory $\mathbb{T}$ over any signature whatsoever provided $\mathbb{T}$ has up to isomorphism exactly one model in every Grothendieck topos $\mathcal{E}$. E.g. a slight modification of the inconsistent theory $\{\top\vdash\bot\}$ over the empty signature, namely adding a sort symbol $O$ and “contextualising” $\mathbb{T}_1'=\{\top\vdash_{x:O}\bot\}$ has models exactly the initial objects but these are unique (and every topos has one) whence $Set[\mathbb{T}_1']\simeq Set$.

Note also the difference in behavior between the inconsistent and the empty theory with respect to enlargening the signature: adding a sort symbol does not change the categories of models of the inconsistent theory up to isomorphism whereas $\mathbb{T}_1'$ has entirely different categories of models from the empty theory over the signature containing a sort symbol $O$ since the latter is the theory of objects $\mathbb{O}$; but $\mathbb{T}_1'$ is a quotient of $\mathbb{O}$ whence we can think of $\mathbb{T}_1'$ as an axiomatisation of $Set$ as subtopos of $Set[\mathbb{O}]$ (see at level for further information on the duality between quotient theories and subtoposes of the classifying topos). To sum up “paraphrasing” Tolstoy: there is only one way to be inconsistent but an infinite number of ways of being empty!

## Generalizations

In the context of (∞,1)-topos theory there is a notion of n-localic (∞,1)-topos.

Notice that a locale is itself a (Grothendieck) (0,1)-topos. Hence a localic topos is a 1-topos that behaves essentially like a (0,1)-topos. In the wider context this would be called a 1-localic (1,1)-topos.

## References

1. The ‘etendu’ in the title of Lawvere’s monograph might not be a misspelled noun, but an adjective as part of a back translation of a (hypothetical) French expression ‘ensembles étendus’. See this nForum thread for some discussion and speculation on this point.

Last revised on May 19, 2020 at 10:36:18. See the history of this page for a list of all contributions to it.