# A Grothendieck site is a small category C equipped with a Grothendieck topology T. A Grothendieck topology T consists of a collection of subfunctors

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1 Contents 5 Grothendieck topologies 1 6 Exactness properties 10 7 Geometric morphisms 17 8 Points and Boolean localization 22 5 Grothendieck topologies A Grothendieck site is a small category C equipped with a Grothendieck topology T. A Grothendieck topology T consists of a collection of subfunctors R hom(,u), U C, called covering sieves, such that the following hold: 1) (base change) If R hom(,u) is covering and φ : V U is a morphism of C, then φ 1 (R) = {γ : W V φ γ R} is covering for V. 2) (local character) Suppose R,R hom(,u) and R is covering. If φ 1 (R ) is covering for all φ : V U in R, then R is covering. 3) hom(,u) is covering for all U C. 1

2 Typically, Grothendieck topologies arise from covering families in sites C having pullbacks. Covering families are sets of maps which generate covering sieves. Suppose that C has pullbacks. A topology T on C consists of families of sets of morphisms {φ α : U α U}, U C, called covering families, such that 1) Suppose φ α : U α U is a covering family and ψ : V U is a morphism of C. Then the set of all V U U α V is a covering family for V. 2) Suppose {φ α : U α V } is covering, and {γ α,β : W α,β U α } is covering for all α. Then the set of composites is covering. W α,β γ α,β U α φ α U 3) The singleton set {1 : U U} is covering for each U C. 2

3 Examples: 1) X = topological space. The site op X is the poset of open subsets U X. A covering family for an open U is an open cover V α U. 2) X = topological space. The site loc X is the category of all maps f : Y X which are local homeomorphisms. f : Y X is a local homeomorphism if each x Y has a neighbourhood U such that f (U) is open in X and the restricted map U f (U) is a homeomorphism. A morphism of loc X is a commutative diagram Y g Y f X where f and f are local homeomorphisms. A family {φ α : Y α Y } of local homeomorphisms (over X) is covering if X = φ α (Y α ). 3) S = a scheme (topological space with sheaf of rings locally isomorphic to affine schemes Sp(R)). The underlying topology on S is the Zariski topology. f 3

4 The Zariski site Zar S is the poset with objects all open subschemes U S. A family V α U is covering if V α = U (as sets). A scheme homomorphism φ : Y S is étale at y Y if a) O y is a flat O f (y) -module (φ is flat at y). b) φ is unramified at y: O y /M f (y) O y is a finite separable field extension of k( f (y)). Say that a map φ : Y S is étale if it is étale at every y Y (and locally of finite type). 4) S = scheme. The étale site et S has as objects all étale maps φ : V S and all diagrams V V for morphisms (with φ,φ étale). φ S An étale cover is a collection of étale morphisms φ α : V α V such that V = φ α (V α ). φ Equivalently every morphism Sp(Ω) V lifts to some V α if Ω is a separably closed field. 4

5 5) The Nisnevich site Nis S has the same underlying category as the étale site, namely all étale maps V S and morphisms between them. A Nisnevich cover is a family of étale maps V α V such that every morphism Sp(K) V lifts to some V α where K is any field. 6) A flat cover of a scheme T is a set of flat morphisms φ α : T α T (ie. mophisms which are flat at each point) such that T = φ α (T α ) as a set (equivalently T α T is faithfully flat). (Sch S ) f l is the big flat site. Here s a trick: pick a large cardinal κ; then (Sch S ) is the category of S-schemes X S such that the cardinality of both the underlying point set of X and all sections O X (U) of its sheaf of rings are bounded above by κ. 7) There are corresponding big sites (Sch S ) Zar, (Sch S ) et, (Sch S ) Nis,... and you can play similar games with topological spaces. 5

6 8) Suppose that G = {G i } is profinite group such that all G j G i are surjective group homomorphisms. Write also G = lim G i. A discrete G-set is a set X with G-action which factors through an action of G i for some i. G Set d f is the category of G-sets which are both discrete and finite. A family U α X is covering if and only if U α X is surjective. Main example: G is the profinite group {G(L/K)} of Galois groups of the finite Galois extensions L/K of a field K. 9) Suppose that C is a small category. Say that R hom(,x) is covering if and only if 1 x R. This is the chaotic topology on C. 10) Suppose that C is a site and that U C. The slice category C /U inherits a topology from C : the set of maps V α V U covers V U if and only if the family V α V covers V. 6

7 Definitions: Suppose that C is a Grothendieck site. 1) A presheaf (of sets) on C is a functor C op Set. If A is a category, an A -valued presheaf on C is a functor C op A. The set-valued presheaves on C form a category (morphisms are natural transformation), written Pre(C ). One defines presheaves taking values in any category: I write spre(c ) for presheaves on C taking values in simplicial sets this is the category of simplicial presheaves on C. 2) A sheaf (of sets) on C is a presheaf F : C op Set such that the canonical map F(U) lim V U R F(V ) is an isomorphism for each covering sieve R hom(,u). Morphisms of sheaves are natural transformations: write Shv(C ) for the corresponding category. The sheaf category Shv(C ) is a full subcategory of Pre(C ). 7

8 One defines sheaves in any complete category, such as simplicial sets: s Shv(C ) denotes the category of simplicial sheaves on the site C. Exercise: If the topology on C is defined by a pretopology (so that C has all pullbacks), show that F is a sheaf if and only if all pictures F(U) α F(U α ) α,β F(U α U U β ) arising from covering families U α U are equalizers. Lemma ) If R R hom(,u) and R is covering then R is covering. 2) If R,R hom(,u) are covering then R R is covering. Proof. 1) φ 1 (R) = φ 1 (R ) for all φ R. 2) φ 1 (R R ) = φ 1 (R ) for all φ R. Suppose that R hom(,u) is a sieve, and F is a presheaf on C. Write F(U) R = lim V U R F(V ) I say that F(U) R is the set of R-compatible families in U. 8

9 If S R then there is an obvious restriction map Write F(U) R F(U) S LF(U) = lim R F(U) R where the colimit is indexed over the filtering diagram of all covering sieves R hom(,u). Then x LF(U) is a presheaf and there is a natural presheaf map η : F LF Say that a presheaf G is separated if (equivalently) 1) the map η : G LG is monic in each section, ie. all functions G(U) LG(U) are injective, or 2) Given x,y G(U), if there is a covering sieve R hom(,u) such that φ (x) = φ (y) for all φ R, then x = y. Lemma ) LF is separated, for all F. 2) If G is separated then LG is a sheaf. 3) If f : F G is a presheaf map and G is a sheaf, then f factors uniquely through a presheaf map f : LF G. Proof. Exercise. 9

10 Corollary ) The object L 2 F is a sheaf for every presheaf F. 2) The functor F L 2 F is left adjoint to the inclusion Shv(C ) Pre(C ). The unit of the adjunction is the composite One often writes for the composite (5.1). F η LF η L 2 F (5.1) η : F L 2 F = F L 2 F = F is the associated sheaf for F, and η is the canonical map. 6 Exactness properties Lemma ) The associated sheaf functor preserves all finite limits. 2) Shv(C ) is complete and co-complete. Limits are formed sectionwise. 3) Every monic is an equalizer. 4) If θ : F G in Shv(C ) is both monic and epi, then θ is an isomorphism. 10

11 Proof. 1) LF is defined by filtered colimits, and finite limits commute with filtered colimits. 2) If X : I Shv(C ) is a diagram of sheaves, then the colimit in the sheaf category is L 2 (lim X), where lim X is the presheaf colimit. 3) If A X is a subset, then there is an equalizer A X p X/A The same holds for subobjects A X of presheaves, and hence for subobjects of sheaves, since L 2 is exact. 4) The map θ appears in an equalizer F θ G since θ is monic. θ is an epi, so f = g. But then 1 G : G G factors through θ, giving a section σ : G F. Finally, θσθ = θ and θ is monic, so σθ = 1. Definitions: 1) A presheaf map f : F G is a local epimorphism if for each x G(U) there is a covering sieve R hom(,u) such that φ (x) = f (y φ ) for all φ : V U in R. f g K 11

12 2) f : F G is a local monic if given x,y F(U) such that f (x) = f (y), then there is a covering sieve R hom(,u) such that φ (x) = φ (y) for all φ : V U in R. 3) A presheaf map f : F G which is both a local epi and a local monic is a local isomorphism. Lemma ) The natural map η : F LF is a local monomorphism and a local epimorphism. 2) Suppose that f : F G is a presheaf morphism. Then f induces an isomorphism of associated sheaves if and only if f is both a local epi and a local monic. Proof. For 2) observe that, given a commutative diagram F g F f h F of presheaf morphisms, if any two of f,g and h are local isomorphisms, then so is the third. A sheaf map g : E E is a monic (respectively epi) if and only if it is a local monic (respectively local epi). 12

13 A Grothendieck topos is a category E which is equivalent to a sheaf category Shv(C ) on some Grothendieck site C. Grothendieck toposes are characterized by exactness properties: Theorem 6.3 (Giraud). A category E having all finite limits is a Grothendieck topos if and only if it has the following properties: 1) E has all small coproducts; they are disjoint and stable under pullback 2) every epimorphism of E is a coequalizer 3) every equivalence relation R E E in E is a kernel pair and has a quotient 4) every coequalizer R E Q is stably exact 5) there is a (small) set of objects which generates E. A sketch proof of Giraud s Theorem appears below, but the result is proved in many places see, for example, [2], [3]. See also [1]. Here are the definitions of the terms appearing in the statement of the Theorem: 13

14 1) The coproduct i A i is disjoint if all diagrams /0 A i A j i A i are pullbacks for i j. i A i is stable under pullback if all diagrams i B B A i i A i are pullbacks. B 3) An equivalence relation is a monomorphism m = (m 0,m 1 ) : R E E such that a) the diagonal : E E E factors through m (ie. a a) b) the composite R m E E τ E E factors through m (ie. a b b a). c) the map B (m 0 m 0,m 1 m 1 ) : R E R R R factors through m (this is transitivity) where 14

15 the pullback is defined by R E R m 1 R m 0 m 0 R m1 E The kernel pair of a morphism u : E D is a pullback E m 0 R m 1 E u u D (Exercise: every kernel pair is an equivalence relation). A quotient for an equivalence relation (m 0,m 1 ) : R E E is a coequalizer R m 0 m 1 E E/R 4) A coequalizer R E Q is stably exact if the diagram R Q Q E Q Q Q is a coequalizer for all morphisms Q Q. 5) A generating set is a set {A i } which detects non-trivial monomorphisms: if a monomorphism m : P Q induces bijections hom(a i,p) hom(a i,q) for all i, then m is an isomorphism. 15

16 Exercise: Show that any category Shv(C) on a site C satisfies the conditions of Giraud s theorem. The family L 2 hom(,u), U C is a set of generators. Sketch proof of Giraud s Theorem. The key is to show that the category E is cocomplete see [2]. If A is the set of generators for E prescribed by Giraud s theorem, let C be the full subcategory of E on the set of objects A. A subfunctor R hom(,x) on C is covering if the map y x y x R is an epimorphism of E. Every object E E represents a sheaf hom(,e) on C, and a sheaf F on C determines an object of E. The adjunction lim hom(,y) F hom( lim y,e) = hom(f,hom(,e)) hom(,y) F determines an adjoint equivalence between E and Shv(C ). 16 y

17 The proof of Giraud s Theorem is arguably more important than the statement of the theorem itself. Here are some examples of the use of the basic ideas: 1) Suppose that G is a sheaf of groups, and let G Shv(C ) denote the category of all sheaves X admitting G-action, with equivariant maps between them. G Shv(C ) is a Grothendieck topos, called the classifying topos for G, by Giraud s theorem. The objects G hom(,u) form a generating set. 2) If G = {G i } is a profinite group with all transition maps G i G j epi, then the category G Set d of discrete G-sets is a Grothendieck topos. The finite discrete G-sets form a generating set for this topos, and the site of finite discrete G-sets is the site prescribed by Giraud s theorem. 7 Geometric morphisms Suppose that C and D are Grothendieck sites. A geometric morphism f : Shv(C ) Shv(D) consists of functors f : Shv(C ) Shv(D) and f : Shv(D) Shv(C ) such that 17

18 1) f is left adjoint to f, and 2) f preserves finite limits. The left adjoint f is called the inverse image functor, while f is called the direct image. The inverse image f is left and right exact in the sense that it preserves all finite limits and colimits. The direct image f is usually not left exact (does not preserve finite colimits), and hence has higher derived functors. Examples 1) Suppose f : X Y is a continuous map of topological spaces. Pullback along f induces a functor op Y op X : U Y f 1 (U). Open covers pull back to open covers, so if F is a sheaf on X then composition with the pullback gives a sheaf f F on Y with f F(U) = F( f 1 (U)). The resulting functor f : Shv(op X ) Shv(op Y ) is the direct image The left Kan extension f p : Pre(op Y ) Pre(op X ) is defined by f p G(V ) = lim G(U) 18

19 where the colimit is indexed over all diagrams V U X f Y The category op Y has all products (ie. intersections), so the colimit is filtered. The functor G f p G therefore commutes with finite limits. The inverse image functor f : Shv(op Y ) Shv(op X ) is defined by f (G) = L 2 f p (G). The resulting pair of functors f, f forms a geometric morphism f : Shv(op X ) Shv(op Y ). 2) Suppose f : X Y is a morphism of schemes. Etale maps (resp. covers) are stable under pullback, and so there is a functor et Y et X defined by pullback, and if F is a sheaf on et X then there is a sheaf f F on et Y defined by f F(V Y ) = F(X Y V X). The restriction functor f : Pre(et X ) Pre(et Y ) has a left adjoint f p defined by f p G(U X) = lim G(V Y ) 19

20 where the colimit is indexed over all diagrams U V X f Y where both vertical maps are étale. The colimit is filtered, because étale maps are stable under pullback and composition. The inverse image functor f : Shv(et Y ) Shv(et X ) is defined by f F = L 2 f p F, and so f induces a geometric morphism f : Shv(et X ) Shv(et Y ). A morphism of schemes f : X Y induces a geometric morphism f : Shv(? X ) Shv(? Y ) and/or f : (Sch X )? (Sch Y )? for all of the geometric topologies (eg. Zariski, flat, Nisnevich, qfh,...), by similar arguments. 3) A point of Shv(C ) is a geometric morphism Set Shv(C ). Every point x X of a topological space X determines a continuous map {x} X and hence a geometric morphism Set = Shv(op {x} ) x Shv(op X ) 20

21 The set x F = lim F(U) x U is the stalk of F at x. The object x Z associated to a set Z is called a skyscraper sheaf. 4) Suppose that k is a field. A scheme map x : Sp(k) X induces a geometric morphism Shv(et k ) Shv(et X ) If k happens to be separably closed, then there is an equivalence Shv(et k ) Set and the resulting geometric morphism x : Set Shv(et X ) is called a geometric point of X. The inverse image functor F f F = lim F(U) U is the stalk of F at x. Sp(k) x 5) Suppose that S and T are topologies on a site C so that S T. In other words, T has more covers than S and hence refines S. Then every sheaf for T is a sheaf for S. Write X π : Shv(C,T ) Shv(C,S) 21

22 for the corresponding inclusion. The associated sheaf functor for the topology T gives a left adjoint π for the inclusion functor π, and π preserves finite limits. Example: There is a geometric morphism Shv(C ) Pre(C ) determined by the inclusion of the sheaf category in the presheaf category and the associated sheaf functor. 8 Points and Boolean localization A Grothendieck topos Shv(C ) has enough points if there is a set of geometric morphisms x i : Set Shv(C ) such that the induced morphism is faithful. Shv(C ) (x i ) Set i 22

23 Lemma 8.1. Suppose that f : Shv(D) Shv(C ) is a geometric morphism. Then the following are equivalent: a) f : Shv(C ) Shv(D) is faithful. b) f reflects isomorphisms c) f reflects epimorphisms d) f reflects monomorphisms Proof. Suppose that f is faithful, ie. that f (g 1 ) = f (g 2 ) implies that g 1 = g 2. Suppose that m : F G is a morphism of Shv(C ) such that f (m) is monic. If m f 1 = m f 2 then f ( f 1 ) = f ( f 2 ) so f 1 = f 2. The map m is therefore monic. Similarly, f reflects epimorphisms and hence reflects isomorphisms. Suppose that f reflects epis and suppose given g 1,g 2 : F G such that f (g 1 ) = f (g 2 ). g 1 = g 2 if and only if their equalizer e : E F is an isomorphism. But f preserves equalizers and reflects isomorphisms, so e is an epi and g 1 = g 2. The other arguments are similar. 23

24 Here are some basic definitions: 1) A lattice L is a partially ordered set which has coproducts x y and products x y. 2) A lattice L has 0 and 1 if it has an initial and terminal object, respectively. 3) A lattice L is said to be distributive if for all x,y,z. x (y z) = (x y) (x z) 4) Suppose that L is a lattise with 0 and 1 and that x L. A complement for x is an element a such that x a = 1 and x a = 0. If L is also distibutive the complement, if it exists, is unique: if b is another complement for x, then b = b 1 = b (x a) = (b x) (b a) = (x a) (b a) = (x b) a = a One usually writes x for the complement of x. 5) A Boolean algebra B is a distributive lattice with 0 and 1 in which every element has a complement. 6) A lattice L is said to be complete if it has all small limits and colimits (aka. all small meets and joins). 24

25 7) A frame P is a lattice that has all small joins and satisfies an infinite distributive law U ( i V i ) = i (U V i ) Examples: 1) The poset O(T ) of open subsets of a topological space T is a frame. 2) The power set P(I) of a set I is a complete Boolean algebra. 3) Every complete Boolean algebra B is a frame. In effect, every join is a filtered colimit of finite joins. Every frame A has a canonical Grothendieck topology: a family y i x is covering if i y i = x. Write Shv(A) for the corresponding sheaf category. Every complete Boolean algebra B is a frame, and has an associated sheaf category Shv(B). 25

26 Example: Suppose that I is a set. Then there is an equivalence Shv(P(I)) Set i I Any set I of points x i : Set Shv(C ) assembles to give a geometric morphism x : Shv(P(I)) Shv(C ). Here x(f i ) = x i (F i ). i I Lemma 8.2. Suppose that F is a sheaf of sets on a complete Boolean algebra B. Then the poset Sub(F) of subobjects of F is a complete Boolean algebra. Proof. Sub(F) is a frame, by an argument on the presheaf level. It remains to show that every object G Sub(F) is complemented. The obvious candidate for G is G = H H G=/0 and we need to show that G G = F. Every K hom(,a) is representable: in effect, K = lim hom(,b) = hom(,c) hom(,b) K 26

27 where C = hom(,b) K B B. It follows that Sub(hom(,A)) = Sub(A) is a complete Boolean algebra. Consider all diagrams φ 1 (G) G hom(,a) φ F There is an induced pullback φ 1 (G) φ 1 (G) G G = hom(,a) F is a union of its representables (all φ are monic since all hom(,a) are subobjects of the terminal sheaf), so G G = F. Lemma 8.3. Suppose that B is a complete Boolean algebra. Then every epimorphism π : F G in Shv(B) has a section. Remark 8.4. Lemma 8.3 asserts that the sheaf category on a complete Boolean algebra satisfies the Axiom of Choice. 27 φ F

28 Proof of Lemma 8.3. Consider the family of lifts N This family is non-empty, because every x G(1) restricts along some covering B 1 to a family of elements x B which lift to F(B). All maps hom(,b) G are monic, since all maps hom(,b) hom(,1) = are monic. Thus, all such morphisms represent objects of Sub(G), which is a complete Boolean algebra by Lemma 8.2. Zorn s Lemma implies that the family of lifts has maximal elements. Suppose that N is maximal and that N /0. Then there is an x N(C) for some C, and there is a cover B C such that x B N(B ) lifts to F(B ) for all B. Then N hom(,b ) = /0 so the lift extends to a lift on N hom(,b ), contradicting the maximality of N. A Boolean localization for Shv(C ) is a geometric morphism p : Shv(B) Shv(C ) such that B is a complete Boolean algebra and p is faithful. F G π 28

29 Theorem 8.5 (Barr). Boolean localizations exist for every Grothendieck topos Shv(C ). Theorem 8.5 is one of the big results of topos theory, and is proved in multiple places see [2], for example. There is a relatively simple description of the proof in [1]. In general, a Grothendieck topos Shv(C ) does not have enough points (eg. sheaves on the flat site for a scheme), but Theorem 8.5 asserts that every Grothendieck topos has a fat point given by a Boolean localization. References [1] J.F. Jardine. Local Homotopy Theory. Springer Monographs in Mathematics. Springer-Verlag, New York, [2] Saunders Mac Lane and Ieke Moerdijk. Sheaves in geometry and logic. Universitext. Springer-Verlag, New York, A first introduction to topos theory, Corrected reprint of the 1992 edition. [3] Horst Schubert. Categories. Springer-Verlag, New York, Translated from the German by Eva Gray. 29

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