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added to locale a section relation to toposes stating localic reflection
thanks, fixed.
Added to locale remakrs on how the frame of subobjects $Sub_{\mathcal{E}}(*) \simeq \tau_{\leq -1}$ is the subcategory of $(-1)$-truncated objects and how this is the beginning of a pattern continued at n-localic (infinity,1)-topos.
Have added to locale in the section Category of locales two theorems from the Elephant on the externalization of internal locales (needed in the discussion of Bohrification)
At locale in the section relation to topological spaces I have tried to make some of the statements more pronounced.
I have made more explicit in the discussion of localic reflection at locale that $Sh : Locale \to Topos$ is fully faithful in fact as a 2-functor.
The discussion of this point in the entry would deserve a bit of expansion. Maybe I find time to expand on it later.
I realized that we never stated the definition of the 2-category (or (1,2)-category) of locales. So I have now added it to the Definition-section at Locale.
Oh. dear, I should take a break :-) I did look for it, but didn’t see it! Thanks.
Anyway, I have now added to Locale, too, the precise definition, and some more pointers.
Added to locale the observation that frames are the same as lex total posets, by way of introducing connections between locales and toposes.
Nice, thanks! Should we have a page lex-total category with Street’s theorem on it?
The second paragraph of Locale says:
For example, there is a locale of all surjections from natural numbers (thought of as forming the discrete space $N$) to real numbers (forming the real line $R$): the locale of real numbers. This local has no points, since there are no such surjections, but it contains many nontrivial open subspaces; these open subspaces are generated by a family parametrised by $n\colon N$ and $x\colon R$; the basic open associated to $n$ and $x$ may be described as $\{f\colon N \twoheadrightarrow R \;|\; f(n) = x\}$.
Can someone explain exactly what this means, or at least give a reference?
Capitalised ’Locale’ is redirected to Loc, the category. So you mean the lower case version locale.
Does the description at locale of real numbers help? It seems to be largely the work of Toby.
Yes I mean locale. The locale of real numbers doesn’t help at all. In any case I would guess that the particular sets (locales?) $N$ and $R$ are basically irrelevant for this example. (Except for having $|N|\lt|R|$, which is supposed to imply there are no points.) I just want to know what definition is underlying this example.
Here’s my speculation, though it doesn’t seem to work right. Let $X$ and $Y$ be sets, and let $\mathcal{P}$ be the poset of “finite partial graphs” in $X\times Y$, i.e., finite subsets $\Gamma\subseteq X\times Y$ such that the composite $\Gamma\to X\times Y \to X$ is injective. Let $\mathcal{O}$ be the collection of upward-closed subsets of $\mathcal{P}$: this set is supposed to be the collection of “opens” in the locale.
Given any function $f\colon X\to Y$ let $U_f\in \mathcal{O}$ be the set of all $\Gamma\in \mathcal{P}$ such that $\Gamma\cup \Gamma_f$ is not the graph of any function: it’s the set of finite partial graphs inconsistent with $f$. Note that $U_f\neq \mathcal{P}$ since $\varnothing\notin U_f$, and I can show that $U_f$ is non-trivially an intersection of non-maximal elements of $\mathcal{O}$ if and only if $f$ is not surjective. That is, surjective functions give rise to points in the locale $\mathcal{O}$. What I can’t prove is that these are the only points in the locale.
Edit. Nevermind that doesn’t make sense anyway. But I expect they mean some kind of example along these lines.
The locale of surjections $X\to Y$ is the classifying locale of the propositional geometric theory of surjections $X\to Y$. There are basic propositions “$f(x)=y$” (an atomic formula) for each $x\in X$ and $y\in Y$, and then axioms like $\vdash_{x\in X} \bigvee_{y\in Y} f(x)=y$ and $(f(x)=y) \wedge (f(x)=y') \vdash_{x\in X,y\neq y'\in Y} \bot$ to make it a function and $\vdash_{y\in Y} \bigvee_{x\in X} f(x)=y$ to make it surjective. So the opens of the locale are generated by these propositions “$f(x)=y$” under finite meet and arbitrary join, modulo those axioms. This can be re-expressed in terms of partial functions in a way kind of like you say, but I don’t remember (and don’t have time to work out) exactly how it goes. But with this description, the points of the locale are, by the universal property of classifying locales (or equivalently, by the universal property of a frame presented by generators and relations), precisely surjections $X\to Y$.
Thanks Mike. I would still love a reference for this. It looks to me that what I was describing above is the locale of “partial graphs in $X\times Y$”; at least, the points are precisely the partial graphs. I assume the additional axioms are imposed by choosing appropriate sublocales.
It’s C1.2.8 in Sketches of an Elephant.
Thanks Mike.
Here’s a brief sketch of the construction (where I’ve unwound most of the terminology Johnstone uses here).
Fix discrete spaces $X$ and $Y$, and let $M=\prod_X Y$ with the usual product topology. For a finite subset $S\subseteq Y$ write $M_S=\{f\in M\;|\; S\subseteq f(X)\}$, the subset of functions which map onto $S$. Let $\mathcal{O}$ be the collection of open subsets $U\subseteq M$ such that
for all open $V\subseteq M$ and finite $S\subseteq Y$, we have that $V\cap M_S\subseteq U$ implies $V\subseteq U$.
The claims are that:
$\mathcal{O}$ is a complete lattice satisfying the infinite distributive law, i.e., it corresponds to a locale.
If $X$ is infinite then $\varnothing\in \mathcal{O}$ and $M\in \mathcal{O}$, e.g., $\mathcal{O}$ is a non-trivial locale when $X$ is infinite and $Y$ is non-empty.
The points of $\mathcal{O}$ correspond exactly to surjective functions $X\to Y$.
In particular, if $\infty\leq |X| \lt |Y|$ then $\mathcal{O}$ is a non-trivial pointless locale. (Johnstone in the example assumes $X=\mathbb{N}$, but as far as I can tell the construction is entirely general.)
It is easy to see that $\mathcal{O}$ is closed under pairwise intersections of open sets and contains $M$, so has pairwise meets and a top element. To show it is a locale, you need to know that for every open set $U$ there is a smallest open set $j(U)$ which contains $U$ and is an element of $\mathcal{O}$. The recipe for $j$ is to define for any open set $U$,
$h(U) := \bigcup_{V\cap M_S\subseteq U} V,$where the union ranges over all open $V\subseteq U$ and finite $S\subseteq Y$ satisfying the condition. Then iterate $h$ some possibly transfinite number of times to get $j(U)$. Then $j(\varnothing)$ is the bottom element and $\bigvee U_i = j(\bigcup U_i)$. The key observation is that $h$ preserves pairwise intersection and has exactly elements of $\mathcal{O}$ as its fixed points, whence $j$ has these same properties.
More conceptually: $\mathcal{O}$ is the limit in Locale of the family of subspaces $M_S$ of $M$ indexed by the poset of all finite $S\subseteq Y$; compare with the limit in Top, which is just the subspace $\bigcap_S M_S$ of surjective functions. (Johnntone says “intersection of sublocales” here, but I think it is also an example of a limit in Locale this context, as is the intersection subspace in Top.)
Unrelated remark: Apparently the TeX engine on this thing displays the same character for $\backslash varnothing$ ($\varnothing$) and $\backslash emptyset$ ($\emptyset$), which is unfortunate since $\emptyset$ is so ugly.
Thanks for writing that out! It would be useful to add it to some nlab page, maybe locale?
I generally prefer \emptyset
to \varnothing
; I find the latter uglier than the former. But if you really want the latter you may be able to get it with unicode ∅
(∅).
I added a bit about the locale of surjections discussed in comments #13–#21, only at the level of detail of Mike #17 (actually less detail than that), not Charles #20.
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