nLab Artin gluing

Idea

Artin gluing is a fundamental construction in locale theory and topos theory. The original example is the way in which a topological space or locale $X$ may be glued together from an open subspace $i \colon U \hookrightarrow X$ and its closed complement $j \colon K \hookrightarrow X$. The analogous construction for toposes gives the sheaf topos $Sh(X)$ via a gluing together of $Sh(U)$ and $Sh(K)$, and applies more generally to give a sense of how to put two toposes together so that one becomes an open subtopos and the other a closed subtopos of the gluing.

The topological case

Let us consider first the case of topological spaces. Let

• $X$ be a topological space,

• $i \colon U \hookrightarrow X$ an open subspace, and

• $j \colon K \hookrightarrow X$ the complementary closed subspace.

Let $O(X)$ denote the topology of $X$. There is an injective map

$$\array{ \langle i^\ast, j^\ast \rangle &\colon\;& O(X) &\longrightarrow& O(U) \times O(K) \\ && V &\mapsto& (U \cap V, K \cap V) }$$

that is a homomorphism of frames. The general problem is to characterize the image of this map: in terms of structure pertaining to $O(U)$ and $O(K)$, which pairs $(W, W')$ of relatively open sets in $U$ and $K$ “glue together” to form an open set $W \cup W'$ in $X$?

Let $int_X \colon P(X) \to P(X)$ denote the interior operation, assigning to a subset of $X$ its interior. This is a left exact comonad on $P(X)$. (Indeed, topologies on the set $X$ are in natural bijection with left exact comonads on $P(X)$.) Our problem is to understand when the inclusion

$$W \cup W' \;\xhookrightarrow{\phantom{--}}\; int_X(W \cup W')$$

obtains. Since $W \in O(U)$ is already open when considered as a subset of $X$, this condition boils down to the condition that

(1)$$W' \;\xhookrightarrow{\phantom{--}}\; int_X(W \cup W')$$

Proposition

A necessary and sufficient condition for (1) is that the inclusion $W' \hookrightarrow int_X(W \cup K)$ obtains.

Proof

The necessity is clear since $W' \subseteq K$. The sufficiency is equivalent to having an inclusion

$$W' \cap int_X(W \cup K) \;\xhookrightarrow{\phantom{--}}\; int_X(W \cup W') \,.$$

Since $W'$ is relatively open in the subspace $K$, we may write $W' = K \cap V$ for some $V \in O(X)$, and so we must check that there is an inclusion

$$(K \cap V) \cap int_X(W \cup K) \;\xhookrightarrow{\phantom{--}}\; int_X(W \cup (K \cap V))$$

or in other words – using distributivity and the fact that $int_X$ preserves intersections – an inclusion

$$K \cap V \cap int_X(W \cup K) \;\xhookrightarrow{\phantom{--}}\; int_X(W \cup K) \cap int_X(W \cup V) \,.$$

But this is clear, since we have

$$K \cap V \cap int_X(W \cup K) \;\xhookrightarrow{\phantom{--}}\; int_X(W \cup K)$$

and

$$K \cap V \cap int_X(W \cup K) \;\xhookrightarrow{\phantom{--}}\; V \;\xhookrightarrow{\phantom{--}}\; W \cup V \;=\; int_X(W \cup V) \,,$$

where to derive the last equation, we use the fact that $W \in O(U)$ and $V$ are open in $X$.

Proposition

The operation

$$O(U) \ni W \;\mapsto\; int_X(W \cup K) \;=\; int_X(W \cup \neg U) \in O(X)$$

is the right adjoint $i_\ast$ to $i^\ast \colon O(X) \to O(U)$.

This is well-known.

Proof

For $V \in O(X)$ we have

$$\frac {V \subseteq int_X(W \cup \neg U) \qquad \text{in} \: O(X)} {V \subseteq W \cup \neg U \qquad \text{in} \: P(X)} \,,$$

but the last condition is equivalent to having $U \cap V \subseteq W$ in $P(X)$, or to $i^\ast(V) = U \cap V \subseteq W$ in $O(X)$.

Summarizing, the gluing condition (1) above (for $W' \in O(K)$, $W \in O(U)$) translates into saying that there is an inclusion

$$W' \;\xhookrightarrow{\phantom{--}}\; j^\ast i_\ast W \,.$$

where $i^\ast, j^\ast$ are restriction maps and $i^\ast \dashv i_\ast$.

For future reference, observe that the operator $j^\ast i_\ast \colon O(U) \to O(K)$ is left exact.

We can turn all this around. Suppose $U$ and $K$ are topological spaces, and suppose $f \colon O(U) \to O(K)$ is left exact. Then we can manufacture a topological space $X$ which contains $U$ as an open subspace and $K$ as its closed complement, and (letting $i$, $j$ being the inclusions as above) such that $f = j^\ast i_\ast$. The open sets of $X$ may be identified with pairs $(W, W') \in O(U) \times O(K)$ such that $W' \subseteq f(W)$; here we are thinking of $(W, W')$ as a stand-in for $W \cup W'$. In particular, open sets $W$ of $U$ give open sets $(W, \varnothing)$ of $X$, while open sets $W'$ of $K$ also give open sets $U \cup W'$ of $X$.

The localic case

The development given above generalizes readily to the context of locales. Thus, let $X$ be a locale, with corresponding frame denoted by $O(X)$. Each element $U \in O(X)$ gives rise to two distinct frames:

• The frame whose elements are algebras (fixed points) of the left exact idempotent monad $U \vee - \colon O(X) \to O(X)$. The corresponding locale is the closed sublocale $\neg U$ (more exactly, the frame surjection $O(X) \to Alg(U \vee -)$ is identified with a sublocale $\neg U \to X$).

• The frame whose elements are algebras of the left exact idempotent monad $U \Rightarrow - \colon O(X) \to O(X)$. (NB: for topological spaces, this is $U \Rightarrow V = int_X(V \cup \neg U)$. This is isomorphic as a frame (but not as a subset of $O(X)$) to the principal ideal of $O(X)$ generated by $U$, which is more obviously the topology of $U$.) The sublocale corresponding to the frame surjection $O(X) \to Alg(U \Rightarrow -)$ is the open sublocale corresponding to $U$.

Put $K = \neg U$, and let $i^\ast: O(X) \to O(U)$, $j^\ast: O(X) \to O(K)$ be the frame maps corresponding to the open and closed sublocales attached to $U$, with right adjoints $i_\ast$, $j_\ast$. Again we have a left exact functor

$$f = j^\ast i_\ast \colon O(U) \to O(K).$$

Observe that this gives rise to a left exact comonad

$$O(U) \times O(K) \to O(U) \times O(K): (W, W') \mapsto (W, W' \wedge f(W)) \qquad (2)$$

whose coalgebras are pairs $(W, W')$ such that $W' \leq f(W)$. The coalgebra category forms a frame.

Theorem

The frame map $\langle i^\ast, j^\ast \rangle \colon O(X) \to O(U) \times O(K)$ is identified with the comonadic functor attached to the comonad (2). In particular, $O(X)$ can be recovered from $O(U)$, $O(K)$, and the comonad (2).

Since $O(U + K) \cong O(U) \times O(K)$, we can think of the frame map $\langle i^\ast, j^\ast \rangle$ as giving a localic surjection $U + K \to X$.

Again, we can turn all this around and say that given any two locales $U$, $K$ and a left exact functor

$$f \colon O(U) \to O(K),$$

we can manufacture a locale $X$ whose frame $O(X)$ is the category of coalgebras for the comonad

$$1_{O(U)} \times \wedge \circ (f \times 1_{O(K)}) \colon O(U) \times O(K) \to O(U) \times O(K) \qquad (3)$$

so that $U$ is naturally identified with an open sublocale of $X$, $K$ with the corresponding closed sublocale, and with a localic surjection $U + K \to X$. This is the (Artin) gluing construction for $f$.

The toposic case

Now suppose given toposes $E$, $E'$ and a left exact functor $\Phi \colon E \to E'$. There is an induced left exact comonad

$$E \times E' \stackrel{\delta \times 1}{\to} E \times E \times E' \stackrel{1 \times \Phi \times 1}{\to} E \times E' \times E' \stackrel{1 \times product}{\to} E \times E' \qquad (3)$$

whose category of coalgebras is again (by a basic theorem of topos theory; see for instance here) a topos, called the Artin gluing construction for $\Phi$, denoted $\mathbf{Gl}(\Phi)$.

Objects of $\mathbf{Gl}(\Phi)$ are triples $(e, e', f \colon e' \to \Phi(e))$. A morphism from $(e_0, e_0^', f_0)$ to $(e_1, e_1^', f_1)$ consists of a pair of maps $g \colon e_0 \to e_1$, $g'\colon e_0^' \to e_1^'$ which respects the maps $f_0, f_1$ :

$$\array{ e_0^' &\overset{f_0}{\to} & \Phi (e_0) \\ g'\downarrow &&\downarrow \Phi(g) \\ e_1^' &\underset{f_1}{\to} & \Phi(e_1) }$$

In other words, the Artin gluing is just the comma category $E' \downarrow \Phi$. (In fact, this comma category is a topos whenever $\Phi$ preserves pullbacks.)

On the other hand, if $E$ is a topos and $U\in E$ is a subterminal object, then it generates two subtoposes that are complements in the lattice of subtoposes, namely, an open subtopos whose reflector is $(-)^U$, and a closed subtopos whose reflector is the pushout $A\mapsto A +_{A\times U} U$. If $E=Sh(X)$ is the topos of sheaves on a locale, then $U$ corresponds to an element of $O(X)$, hence an open sublocale with complement $K$ (say), and the open subtopos can be identified with $Sh(U)$ and the closed one with $Sh(K)$.

Returning to the general case, let us denote the geometric embedding of the open subtopos by $i\colon E_U \hookrightarrow E$ and that of the closed subtopos by $j\colon E_{\neg U}\hookrightarrow E$. Then we have a composite functor, sometimes called the fringe functor,

$$E_U \xrightarrow{i_*} E \xrightarrow{j^*} E_{\neg U}$$

which is left exact.

Theorem

Let $U$ be a subterminal object of a topos $E$, as above. Then the left exact left adjoint

$$\langle i^\ast, j^\ast \rangle \colon E \to E_U \times E_{\neg U}$$

is canonically identified with the comonadic gluing construction $\mathbf{Gl}(j^\ast i_\ast) \to E_U \times E_{\neg U}$. In particular, $E$ can be recovered from $E_U$, $E_{\neg U}$, and the functor $j^* i_*$.

For a proof, see A4.5.6 in the Elephant.

Once again, the import of this theorem may be turned around. If $f \colon E \to F$ is any left exact functor, then the projection

$$\mathbf{Gl}(f) \to E \times F \stackrel{proj}{\to} E$$

is easily identified with a logical functor $\mathbf{Gl}(f) \to \mathbf{Gl}(f)/X$ where $X$ is the subterminal object $(1, 0, 0 \to f(1))$. This realizes $E$ as an open subtopos of $\mathbf{Gl}(f)$. On the other hand, for the same subterminal object $X \hookrightarrow 1$, the corresponding classifying map

$$[X] \colon 1 \to \Omega$$

induces a Lawvere-Tierney topology $j$ given by

$$\Omega \cong 1 \times \Omega \stackrel{[X] \times 1}{\to} \Omega \times \Omega \stackrel{\wedge}{\to} \Omega.$$

Then, the category of sheaves $Sh(j)$, or more exactly the left exact left adjoint $\mathbf{Gl}(f) \to Sh(j)$ to the category of sheaves, is naturally identified with the projection

$$\mathbf{Gl}(f) \to E \times F \stackrel{proj}{\to} F,$$

thus realizing $F$ as equivalent to the closed subtopos (Elephant, A.4.5, pp. 205-206) attached to the subterminal object $X$.

Some details and further adjunctions

The sheaves in $\mathbf{Gl}(f)$ corresponding to the open resp. closed subtoposes can be described explicitly. Recall that the objects of $\mathbf{Gl}(f)$ have the form $(X, Y, u:Y\to f(X))$: then the open copy of $E$ corresponds to the subcategory on those objects $(X, Y, u:Y\to f(X))$ with $u$ an isomorphism in $F$ and the closed copy of $F$ to the subcategory with objects $(X, Y, u:Y\to f(X))$ such that $X\simeq 1$ in $E$.

The open subtopos corresponding to $E$ is dense in $\mathbf{Gl}(f)$ precisely if $f:E\to F$ preserves the initial object since $(0,0,0\to f(0))$ is the initial object in $\mathbf{Gl}(f)$ and $0\to f(0)$ is an isomorphism precisely if $f$ preserves $0$.

To summarize: given a left exact $f\colon E\to F$ we get an open inclusion of $E$ with a further left adjoint:

$$i_\ast \colon E\to \mathbf{Gl}(f) \qquad X\mapsto (X,f(X),id_{f(X)})$$

$$i^\ast\colon \mathbf{Gl}(f)\to E \qquad (X,Y,u)\mapsto X$$

$$i_{!} \colon E\to \mathbf{Gl}(f) \qquad X\mapsto (X,0,0\to f(X)) \quad ,$$

and a closed inclusion of $F$ into $\mathbf{Gl}(f)$ with

$$j_\ast \colon F\to \mathbf{Gl}(f) \qquad X\mapsto (1,X,X\to 1)$$

$$j^\ast\colon\mathbf{Gl}(f)\to F \qquad (X,Y,u)\mapsto Y$$

that will lack the left adjoint $j_!$ in general. The situation when $j_!$ exists is characterized by the following observation:

Proposition

The closed inclusion $j$ is essential i.e. $j^\ast$ has a left adjoint $j_!$ precisely if the fringe functor $f$ has a left adjoint $l$.

Proof

Suppose $j_!$ exists. The fringe functor $f$ is up to natural isomorphism just $j^\ast i_\ast$ and $i^\ast j_!\dashv j^\ast i_\ast$ since adjoints compose.

Conversely, suppose that $l\dashv f$ with $\eta\colon id\to f{l}$ the corresponding unit. Define

$$j_{!} \colon F\to \mathbf{Gl}(f) \qquad Y\mapsto (l(Y),Y,\eta_Y\colon Y\to f{l}(Y)) \quad .$$

Now given morphisms $\alpha\colon Y_1\to Y$ and $u\colon Y\to f(X)$ in $F$ by general properties of a unit there is precisely one morphism $\overline{u\circ\alpha}\colon l(Y_1)\to X$ corresponding to $u\circ\alpha$ under the adjunction such that the following diagram commutes:

$$\array{ Y_1 &\overset{\eta_{Y_1}}{\to} & f{l}(Y_1) \\ \alpha\downarrow &&\downarrow f(\overline{u\circ\alpha}) \\ Y &\underset{u}{\to} & f(X) }$$

This establishes a bijective correspondence between $Y_1\to j^\ast(X,Y,u)$ and $j_!(Y_1)\to (X,Y,u)$ which is natural since $\eta$ is.

In particular, the left adjoint $j_!$ exists if the fringe functor $f$ is the direct image of a geometric morphism, or the inverse image of an essential geometric morphism.

$$j_!\dashv j^\ast\dashv j_\ast \colon \mathbf{Gl}(f)\to F\quad .$$

The existence of a right adjoint for the fringe functor: $f\dashv r\colon F\to E$, on the other hand, corresponds to the existence of an ‘amazing’ right adjoint for the open subtopos inclusion: $i_\ast\dashv i^!:\mathbf{Gl}(f)\to E$.

One direction follows again from the composition of adjoints: $j^\ast i_\ast\dashv i^! j_\ast$ , whereas for the other direction we define:

$$i^!\colon \mathbf{Gl}(f)\to E \qquad (X,Y,u)\mapsto r(Y) \times_{rf(X)} X\quad .$$

When $f$ is fully faithful, the unit $X \to rf(X)$ is an iso, and so we can instead use

$$i^!\colon \mathbf{Gl}(f)\to E \qquad (X,Y,u)\mapsto r(Y)\quad .$$

Note that in this case $E$ is dense and we get a ‘co-cohesive’ adjoint string

$$i_!\dashv i^\ast\dashv i_\ast \dashv i^!\colon \mathbf{Gl}(f)\to E$$

where $i_!$ and $i_\ast$ are fully faithful.

Examples

Since $i:\ast\hookrightarrow E$ is left exact where $\ast$ is the degenerate topos with one identity morphism, every topos $E$ is trivially a result of Artin gluing: $E\simeq E\downarrow i$.

Of course, more interesting examples of the gluing construction abound as well. Here are a few:

• Let $E$ be an (elementary, not necessarily Grothendieck) topos, and let $\hom(1, -): E \to Set$ represent the terminal object $1$ – this of course is left exact. The gluing construction $\mathbf{Gl}(\hom(1, -))$ is called the scone (Sierpinski cone), or the Freyd cover, of $E$.

• If $E$ is a Grothendieck topos and $\Delta \colon Set \to E$ is the (essentially unique) left exact left adjoint, then we have a gluing construction $E \downarrow \Delta$. This gluing may be regarded as the result of attaching a generic open point to $E$.

• A concrete instance of the constructions in both the preceding examples is the Sierpinski topos $Set^{\to}$ corresponding e.g. to $Set\downarrow id_{Set}$: its objects are functions $X\to Y$ between sets $X,Y$ and the closed copy of $Set$ sits on the objects of the form $X\to 1$ and the open copy on the objects $X\overset{\simeq}{\to}Y$.

• Since a topos $\mathcal{E}$ is finitely bicomplete, the product functor $\sqcap:\mathcal{E}\times\mathcal{E}\to\mathcal{E}$ with $(X,Y)\mapsto X\times Y$ is part of an adjoint string $\sqcup\dashv\triangle\dashv\sqcap$ involving the diagonal functor and the coproduct functor. Since $\sqcap$ is left exact, Artin gluing applies. In the case $\mathcal{E}=Set$ , $\mathbb{Gl}(\sqcap)$ yields the topos of hypergraphs; this example is discussed in detail at hypergraph. These cases are somewhat unusual in that the fringe functor here has a left adjoint which itself has a further left adjoint.

Remarks

• Artin gluing for toposes carries over in some slight extra generality, replacing left exact functors $f$ by pullback-preserving functors.

• Artin gluing applies also to other doctrines: regular categories, pretoposes, quasitoposes, etc. See (Carboni-Johnstone) and (Johnstone-Lack-Sobocinski).

Related entries

• Freyd cover

• comma category

• recollement

References

• M. Artin, A. Grothendieck, J. L. Verdier, Théorie des Topos et Cohomologie Etale des Schémas (SGA4), LNM 269 Springer Heidelberg 1972. (exposé IV, sect. 9; in particular: sect. 9.5)

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• Peter Johnstone, Steve Lack, Pawel Sobocinski, Quasitoposes, Quasiadhesive Categories and Artin Glueing , pp.312-326 in LNCS 4624 Springer Heidelberg 2007. (preprint)

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