nLab
point of a topos

Context

Topos Theory

The points of a topos

Definition
Properties
Examples
Related concepts
References

Definition

A point $x$ of a topos $E$ is a geometric morphism

x : Set \overset{\overset{x^{*}}{\leftarrow}}{\underset{x_{*}}{\to}} E

from the base topos Set to $ℰ$ .

For $A \in ℰ$ an object, its inverse image $x^{*} A \in Set$ under such a point is called the stalk of $A$ at $x$ .

If $x$ is given by an essential geometric morphism we say that it is an essential point of $E$ .

Remark

Since Set is the terminal object in the category GrothendieckTopos of Grothendieck toposes, for $ℰ$ a sheaf topos this is a global element of the topos.

Since $Set sime Sh (*)$ is the category of sheaves on the one-point locale, the notion of point of a topos is indeed the direct analog of a point of a locale (under localic reflection).

Definition

A topos is said to have enough points if isomorphy can be tested stalkwise, i.e. if the inverse image functors from all of its points are jointly conservative.

More precisely, $E$ has enough points if for any morphism $f : A \to B$ , we have that if for every point $p$ of $E$ , the morphism of stalks $p^{*} f : p^{*} A \to p^{*} B$ is an isomorphism, then $f$ itself is an isomorphism.

Properties

In presheaf toposes

Proposition

For $C$ a small category, the points of the presheaf topos $[C^{op}, sSet]$ are the flat functors $C \to Set$ :

there is an equivalence of categories

Topos (Set, [C^{op}, Set]) \overset{\overset{}{\leftarrow}}{\underset{}{\to}} FlatFunc (C, Set) .

This appears for instance as (MacLaneMoerdijk, theorem VII 2).

In localic sheaf toposes

For the special case that $E = Sh (X)$ is the category of sheaves on a category of open subsets $Op (X)$ of a topological space $X$ the notion of topos pointscomes from the ordinary notion of points of $X$ .

For notice that

$Set = Sh (*)$ is simply the topos of sheaves on a one-point space.
geometric morphisms $f : Sh (Y) \to Sh (X)$ between sheaf topoi are in a bijection with continuous functions of topological spaces $f : Y \to X$ (denoted by the same letter, by convenient abuse of notation).

It follows that for $E = Sh (X)$ points of $E$ in the sense of points of topoi are in bijection with the ordinary points of $X$ .

The action of the direct image $x^{*} : Set \to Sh (X)$ and the inverse image $x_{*} : Sh (X) \to Set$ of a point $x : Set \to Sh (X)$ of a sheaf topos have special interpretation and relevance:

The direct image of a set $S$ under the point $x : * \to X$ is, by definition of direct image the sheaf

$x_{*} (S) : (U \subset X) \mapsto S (x^{- 1} (U)) = {\begin{matrix} S & if x \in U \\ * & otherwise \end{matrix}$ x_*(S) : (U \subset X) \mapsto S(x^{-1}(U)) = \left\{ \array{ S & if x \in U \\ {*} & otherwise } \right.

This is the skyscraper sheaf ${skysc}_{x} (S)$ with value $S$ supported at $X$ . (In the first line on the right in the above we identify the set $S$ with the unique sheaf on the point it defines. Notice that $S (\emptyset) = pt$ ).
The inverse image of a sheaf $A$ under the point $x : * \to X$ is by definition of inverse image (see the Kan extension formula discussed there), the set

$\begin{aligned} x^{*} (A) & = {colim}_{* \to x^{- 1} (V)} A (V) \\ = {colim}_{V \subset X ∣ x \in V} F (V) \end{aligned} .$ \begin{aligned} x^*(A) & = colim_{{*} \to x^{-1}(V)} A(V) \\ &= colim_{V\subset X| x \in V} F(V) \end{aligned} \,.

This is the stalk of $A$ at he point $x$ ,

$x^{*} (-) = {stalk}_{x} (-) .$ x^*(-) = stalk_x(-) \,.

By definition of geometric morphisms, taking the stalk at $x$ is left adjoint to forming the skyscraper sheaf at $x$ :

for all $S \in Set$ and $A \in Sh (X)$ we have

{Hom}_{Set} ({stalk}_{x} (A), S) ≃ {Hom}_{Sh (X)} (A, {skysc}_{x} (S)) .

In sheaf toposes

The following characterization of points in sheaf toposes a special case of the general statements at morphism of sites.

Proposition

For $C$ a site, there is an equivalence of categories

Topos (Set, Sh (C)) ≃ ConFlatFunc (C, Set) .

This appears for instance as (MacLaneMoerdijk, corollary VII, 4).

Proposition

If $E$ is a Grothendieck topos, there is a small set of points of $E$ which are jointly conservative, and therefore a geometric morphism $Set / X \to E$ , for some set $X$ , which is surjective.

This appears as (Johnstone, lemma 2.2.11, 2.2.12).

(In general, of course, a topos can have a proper class of non-isomorphic points.)

Proposition

A Grothendieck topos has enough points precisely when it underlies a bounded ionad.

In classifying toposes

From the above it follows that if $E$ is the classifying topos of a geometric theory $T$ , then a point of $E$ is the same as a model of $T$ in Set.

Of toposes with enough points

Proposition

If a sheaf topos $E$ has enough points then

there exists a topological space $X$ whose cohomology and homotopy theory is the intrinsic cohomology and intrinsic homtopy theory of the topos;
such that $E$ is the category of equivariant objects in the sheaf topos $Sh (X)$ with respect to some groupoid action on $X$ .

This is due to (Butz) and (Moerdijk).

Examples

General

For $X$ any topological space, the topos of sheaves on (the category of open subsets of) $X$ has enough points: a morphism of sheaves is a mono-/epi-/isomorphism precisely if it is so on every stalk.
Points of over-toposes are discussed at over topos -- points.

Of a local topos

A local topos $(Δ ⊣ Γ dahv coDisc) : E \to Set$ has a canonical point, $(Γ ⊣ coDisc) : Set \to E$ . Morover, this point is an initial object in the category of all points of $E$ (see Equivalent characterizations at local topos.)

Over $∞$ -cohesive sites

Let Diff be a small category version of the category of smooth manifolds (for instance take it to be the category of manifolds embedded in $ℝ^{∞}$ ). Then the sheaf topos $Sh (Diff)$ has precisely one point $p_{n}$ per natural number $n \in ℕ$ , corresponding to the $n$ -ball: the stalk of a sheaf on $Diff$ at that point is the colimit over the result of evaluating the sheaf on all $n$ -dimensional smooth balls.

This is discussed for instance in (Dugger, p. 36) in the context of the model structure on simplicial presheaves.

Toposes with enough points

The following classes of topos have enough points.

every presheaf topos;
every coherent topos;
every Galois topos (see (Zoghaib)).

Related concepts

topological locale

References

Textbook references are section 7.5 of

Saunders MacLane, Ieke Moerdijk, Sheaves in Geometry and Logic

as well as section C2.2 of

Peter Johnstone, Sketches of an Elephant

Carsten Butz, Logical and cohomological aspects of the space of points of a topos (ps)

is a discussion of how for every topos with enough points there is a topological space whose cohomology and homotopy theory is related to the intrinsic cohomology and intrinsic homtopy theoryof the topos.

nLab
point of a topos

Context

Topos Theory

Background

Toposes

Internal Logic

Topos morphisms

Extra stuff, structure, properties

Cohomology and homotopy

In higher category theory

Theorems

The points of a topos

Definition

Definition

Remark

Definition

Properties

In presheaf toposes

Proposition

In localic sheaf toposes

In sheaf toposes

Proposition

Proposition

Proposition

In classifying toposes

Of toposes with enough points

Proposition

Examples

General

Of a local topos

Over $∞$ -cohesive sites

Toposes with enough points

Related concepts

References

nLab point of a topos

Context

Topos Theory

Background

Toposes

Internal Logic

Topos morphisms

Extra stuff, structure, properties

Cohomology and homotopy

In higher category theory

Theorems

The points of a topos

Definition

Definition

Remark

Definition

Properties

In presheaf toposes

Proposition

In localic sheaf toposes

In sheaf toposes

Proposition

Proposition

Proposition

In classifying toposes

Of toposes with enough points

Proposition

Examples

General

Of a local topos

Over ∞-cohesive sites

Toposes with enough points

Related concepts

References

nLab
point of a topos

Over $∞$ -cohesive sites