# Derived Algebraic Geometry IX: Closed Immersions

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1 Derived Algebraic Geometry I: Closed Immersions November 5, 2011 Contents 1 Unramified Pregeometries and Closed Immersions 4 2 Resolutions of T-Structures 7 3 The Proof of Proposition Closed Immersions in Spectral Algebraic Geometry 19 5 Gluing along Closed Immersions 25 6 Gluing Spectral Deligne-Mumford Stacks 30 7 Clutching of Quasi-Coherent Sheaves 35 8 Digression: Local Finite Presentation 40 9 Clutching of Spectral Deligne-Mumford Stacks Unramified Transformations Derived Complex Analytic Geometry Derived Complex Analytic Spaces 61 1

2 Introduction Let C denote the field of complex numbers, and let V denote the category of schemes of finite type over C. The category V admits finite limits but does not admit finite colimits. Given a diagram of finite type C-schemes i j 01 1, 0 we do not generally expect a pushout to exist in the category V. However, there is one circumstance in which we can prove the existence of the pushout = : namely, the case where the maps i and j are both closed immersions. In this case, we can describe the pushout explicitly: the diagram i 01 j 0 1 i is a pushout square in the category of topological spaces (that is, the underlying topological space of is obtained from the underlying topological spaces of 0 and 1 by gluing along 01 ), and the structure sheaf O is given by j O 0 (j i) O 01 i O 1. Our main goal in this paper is to develop an analogous picture in the setting of spectral algebraic geometry. We begin in 1 by introducing a very general notion of closed immersion, which makes sense in the context of T-structured -topoi for an arbitrary pregeometry T (Definition 1.1). To obtain a good theory, we need to assume that the pregeometry T satisfies the additional condition of being unramified in the sense of Definition 1.3. Under this assumption, we will show that the collection of closed immersions behaves well with respect to fiber products (Theorem 1.6). The main step of the proof will be given in 3, using some general approximation results for T-structures which we develop in 2. In 4 we will focus our attention to closed immersions in the setting of spectral algebraic geometry. We will show that the pregeometry T Sp which controls the theory of spectral Deligne-Mumford stacks is unramified (Proposition 4.1). Moreover, we establish a local structure theorem for closed immersions between spectral Deligne-Mumford stacks: if = Spec A is affine, then a map f : Y is a closed immersion if and only if Y Spec B for some A-algebra B for which the underlying map of commutative rings π 0 A π 0 B is surjective (Theorem 4.4). We next study the operation of gluing geometric objects along closed immersions. In 5 we will show that if T is an arbitrary pregeometry and we are given a diagram 0 i 01 of T-structured -topoi where i and j are closed immersions, then there exists a pushout diagram of T- structured -topoi 01 j j (Theorem 5.1). In 6, we specialize to the case where T = T Sp. In this case, we show that if 0, 1, and 01 are spectral Deligne-Mumford stacks, then is also a spectral Deligne-Mumford stack (Theorem 6.1). Suppose we are given a pushout diagram of spectral Deligne-Mumford stacks 01 i j 0 1 i j 2

3 where i and j are closed immersions. In many respects, the geometry of is determined by the geometry of the constituents 0 and 1. For example, in 7, we will show that a quasi-coherent sheaf F QCoh() is canonically determined by its restrictions F 0 = j F QCoh( 0 ) and F 1 = i F QCoh( 1 ), together with a clutching datum i F 0 j F 1 (Theorem 7.1). In 9 we consider a nonlinear analogue of this result: a map of spectral Deligne-Mumford stacks Y is determined by the fiber products Y 0 = Y 0 and Y 1 = Y 1, together with a clutching datum 01 0 Y Y 1 (Theorem 9.1). Moreover, we will show that many properties of the morphism Y can be established by verifying the same property for the induced maps Y 0 0 and Y 1 1 (Proposition 9.3). In order to prove this, we will need some general remarks about finite presentation conditions on morphisms between spectral Deligne-Mumford stacks, which we collect in 8. In the last few sections of this paper, we will apply our general theory of closed immersions to develop a derived version of complex analytic geometry. In 11, we will introduce a pregeometry T an. Roughly speaking, a T an -structure on an -topos is a sheaf O alg of E -rings on equipped with some additional structure: namely, the ability to obtain a new section of O alg by composing a finite collection {s i} 1 i n of sections of O alg with a (possibly only locally defined) holomorphic function Cn C. Our main technical result (Proposition 11.12) asserts that for some purposes (such as forming pullbacks along closed immersions) this additional structure can be ignored. We will deduce this from a more general result about unramified transformations between pregeometries (Proposition 10.3), which we prove in 10. In 12 we will introduce a full subcategory An der C of the -category of T an-structured -topoi, which we will refer to as the - category of derived complex analytic spaces. Using Proposition 11.12, we will show verify that our definition is reasonable: for example, the -category An der C the usual category of complex analytic spaces as a full subcategory (Theorem 12.8). Notation and Terminology admits fiber products (Proposition 12.12), and contains Throughout this paper we will make extensive use of the language of -categories, as developed in [40] and [41] and the earlier papers in this series. For convenience, we will adopt the following reference conventions: (T ) We will indicate references to [40] using the letter T. (A) We will indicate references to [41] using the letter A. (V ) We will indicate references to [42] using the Roman numeral V. (V II) We will indicate references to [43] using the Roman numeral VII. (V III) We will indicate references to [44] using the Roman numeral VIII. Notation 0.1. Recall that L Top denotes the subcategory of Ĉat whose objects are -topoi and whose morphisms are geometric morphisms of -topoi f : Y. We let Top denote the -category L Top op. We will refer to Top as the -category of -topoi. If T is a pregeometry, we let Top(T) denote the -category L Top(T) op, where L Top(T) is defined as in Definition V More informally, the objects of Top(T) are pairs (, O ) where is an -topos and O : T is a T-structure on. A morphism from (, O ) to (Y, O Y ) in Top(T) is given by a geometric morphism of -topoi f : Y together with a local natural transformation f O Y O of T-structures on. Notation 0.2. If R is an E -ring, we let Spec R denote the nonconnective spectral Deligne-Mumford stack Specét R = (, O ), where Shv((CAlgét R) op ) is the -topos of étale sheaves over R, and O is the sheaf of E -rings on given by the formula O (R ) = R. 3

4 1 Unramified Pregeometries and Closed Immersions Let f : (, O ) (Y, O Y ) be a map of schemes. Recall that f is said to be a closed immersion if the following two conditions are satisfied: (a) The underlying map of topological spaces Y determines a homeomorphism from to a closed subset of Y. (b) The map f O Y O is an epimorphism of sheaves of commutative rings on. Our goal in this paper is to develop an analogous theory of closed immersions in the setting of spectral algebraic geometry. With an eye towards certain applications (such as the theory of derived complex analytic spaces, which we study in 11), we will work in a more general setting. Let T be a pregeometry (see Definition V.3.1.1) and let f : (, O ) (Y, O Y ) be a morphism in the -category L Top(T) op of T-structured -topoi. Motivated by the case of classical scheme theory, we introduce the following definitions: Definition 1.1. Let T be a pregeometry, an -topos, and let α : O O be a morphism in Str loc T (). We will say that α is an effective epimorphism if, for every object T, the induced map O() O () is an effective epimorphism in the -topos. We will say that a morphism f : (, O ) (Y, O Y ) in Top(T) is a closed immersion if the following conditions are satisfied: (A) The underlying geometric morphism of -topoi f T ). : Y is a closed immersion (see Definition (B) The map of structure sheaves f O Y O is an effective epimorphism. Remark 1.2. Let T be a pregeometry, and suppose we are given a commutative diagram (, O ) (Y, O Y ) f g h (Z, O Z ) in L Top(T) op. If g is a closed immersion, then f is a closed immersion if and only if h is a closed immersion. In order for Definition 1.1 to be of any use, we need an additional assumption on T. To motivate this assumption, we begin with the following observation: ( ) Suppose we are given a pullback diagram of schemes (, O ) (Y, O Y ) (, O ) f (Y, O Y ). If f is a closed immersion, then the underlying diagram of topological spaces Y Y is also a pullback square. 4

5 We will say that T is unramified if it the analogue of condition ( ) is satisfied for some very simple diagrams of affine T-schemes. The main result of this section (Theorem 1.6) asserts that in this case, one can deduce a version of ( ) for arbitrary T-structured -topoi. Definition 1.3. Let T be a pregeometry. We will say that T is unramified if the following condition is satisfied: for every morphism f : Y in T and every object Z T, the diagram Z Y Z induces a pullback square Z Y Y Z in Top. Y Example 1.4. Let T be a discrete pregeometry. For every object U T, the -topos U is equivalent to S. It follows that T is unramified. Example 1.5. Let T be a pregeometry. Suppose that the composite functor preserves finite products. Then T is unramified. T SpecT Top(T) Top The significance of Definition 1.3 is that it allows us to prove the following result: Theorem 1.6. Let T be an unramified pregeometry, and suppose we are given morphisms f : (Y, O Y ) (, O ) and g : (, O ) (, O ) in Top(T). Assume that f induces an effective epimorphism f O O Y. Then: (1) There exists a pullback square in Top(T). (Y, O Y ) g (Y, O Y ) f f (, O ) g (, O ) (2) The map f induces an effective epimorphism f O O Y in Str loc T (Y ). (3) The underlying diagram of -topoi Y is a pullback square in Top. Y 5

6 (4) For every geometric morphism of -topoi h : Y Z, the diagram h f g O h g O Y h f O h O Y is a pushout square in Str loc T (Z). Corollary 1.7. Let T be an pregeometry and suppose we are given maps (Y, O Y ) f (, O ) (, O ) in L Top(T) op. Assume that T is unramified and that f is a closed immersion. Then there exists a pullback square (Y, O Y ) f (, O ) (Y, O Y ) f (, O ) in L Top(T) op. Moreover, f is a closed immersion and the diagram of -topoi Y Y is a pullback square in L Top op. Proof. Combine Theorem 1.6 with Corollary T It is not difficult to reduce the general case of Theorem 1.6 to the special case in which the -topoi, Y, and are all the same. In 3 we will prove the following result: Proposition 1.8. Let be an -topos and T a pregeometry. Suppose we are given morphisms O 0 β O α O in Str loc T (). Assume that α is an effective epimorphism and that T is unramified. Then: (1) There exists a pushout square σ : O α O β O 0 α 0 O 0 in Str loc T (). (2) The map α 0 is an effective epimorphism. (3) For any geometric morphism of -topoi f : Y, the image f (σ) is a pullback square in Str loc T (Y). Let us assume Proposition 1.8 for the moment and explain how it leads to a proof of Theorem

7 Proof of Theorem 1.6. Choose a pullback diagram Y Y in L Top op. Let p : Top(T) Top be the forgetful functor. We claim that σ can be lifted to a p-limit diagram σ : (Y, O Y ) f (, O ) g (Y, O Y ) f g (, O ) in Top(T 0 ). Using Propositions T and T , it suffices to prove the existence of a pushout square τ : f g O g O Y f O β α O Y in Str loc T (Y ), such that h (τ) remains a pushout square in Str loc T (Z) for every geometric morphism h : Y Z. Proposition 1.8 guarantees the existence of τ. Using Proposition T , we conclude that σ is a pullback square in Top(T 0 ). This proves assertion (1); assertions (3) and (4) follow from the construction. Finally, (2) follows from the corresponding part of Proposition Resolutions of T-Structures Let T be a discrete pregeometry (that is, a small -category which admits finite products). By definition, the -category Str loc T (S) is the full subcategory of Fun(T, S) spanned by those functors which preserve finite products. We can therefore identify Str loc T (S) with the -category P Σ (T op ) studied in T According to Proposition T , the Yoneda embedding T op Str loc T (S) exhibits Str loc T (S) as the -category freely generated from T op by adjoining sifted colimits. In particular, every object of Str loc T (S) can be constructed from objects belonging to the essential image of j by means of sifted colimits (Lemma T ). Our goal in this section is to obtain an analogue of the results cited above, where we replace the - category S of spaces by an arbitrary -topos (and where we do not assume the pregeometry T to be discrete). Roughly speaking, we would like to say that every T-structure on can be resolved by T- structures which are of a particularly simple type (namely, which are elementary in the sense of Definition 2.6). The basic idea is simple: if O Str loc T (), then every global section of O() (where is an object of T) determines a free T-structure on equipped with a map to O. Unfortunately, these maps do not generally give us enough information to recover O, because the objects O(U) generally need not admit many global sections. Consequently, we will not be able to describe O as a colimit of objects of Str T (). We can remedy the situation by considering not only global sections of O, but also local sections over each object. Every map U O() determines a free T-structure on the -topos /U, and a map from this free T-structure to the restriction O U. We will show that O can always be recovered as a relative colimit of these types of maps. To make this idea precise, we need to introduce a bit of terminology. Notation 2.1. Let T be a pregeometry and an -topos. We let Str T () denote the full subcategory of Fun(T, Fun( 1, )) Fun(T,Fun({1},)) 7

8 spanned by those vertices which correspond to an object U and a T-structure on the -topos /U. The -category Str T () comes equipped with a Cartesian fibration p : Str T (). For each object U, the fiber p 1 {U} can be identified with Str T ( /U ). If f : U V is a morphism in, then the associated functor Str T ( /V ) Str T ( /U ) is given by composition with the geometric morphism f : /V /U given by V V V U. The objects of Str T () can be thought of as pairs (U, O), where U and O Str T ( /U ). A morphism from (U, O) to (V, O ) in Str T () can be identified with a pair consisting of a map f : U V in and a map α : O f O in Str T ( /U ). We let Str loc T () be the subcategory of Str T () spanned by all the objects of Str T () together with those morphisms (f, α) in Str T () for which α is local. The functor p restricts to a Cartesian fibration p : Str loc (), whose fiber over an object U can be identified with Strloc T ( /U ). T Let T be a pregeometry and an -topos. According to Proposition V.3.3.1, the -category Str loc T () admits sifted colimits, which are computed pointwise: that is, for each U T, evaluation at U induces a functor Str loc T () which preserves sifted colimits. Our first goal is to establish an analogous statement for colimits relative to the projection p : Str loc T (). Proposition 2.2. Let be an -topos, T a pregeometry, and q : C a Cartesian fibration, where the fibers of q are essentially small sifted -categories. Let q : C be an extension of q which carries the cone point of C to a final object of, and let p : Str loc T () be the forgetful functor of Notation 2.1. Suppose that we are given a map Q : C Str loc T () such that q = p Q and Q carries each q-cartesian morphism in C to a p-cartesian morphism in Str loc T (). Then: (1) There exists a p-colimit diagram Q : C Str loc T () extending Q with p Q = q. (2) For every geometric morphism of -topoi f : Y, the induced map f Q : C Str loc T (Y) is a p -colimit diagram, where p : Str loc T (Y) Y is the projection map. (3) Let Q : C Str loc T () be an arbitrary map extending Q with p Q = q. Then Q is a p-colimit diagram if and only if, for each object U T, the composite map C Str loc T () is a colimit diagram in. The proof of Proposition 2.2 will require the following technical lemma: Lemma 2.3. Let be an -topos and q : C a Cartesian fibration. Assume that each fiber of q is an essentially small sifted -category. Then: (1) Let q : C be another functor equipped with a natural transformation α : q q with the following property: ( ) For every q-cartesian morphism C D in C, the diagram q (C) q (D) q(c) q(d) is a pullback square in. Then q has a colimit in. (2) Let {α i : q i q} i I be a finite collection of natural transformations of functors C which satisfy the hypotheses of (1), and let α : q q be their product (in the -category Fun(C, ) /q ). Then the natural map lim(q ) i I lim (q i) is an equivalence in. 8

9 Proof. We first prove (1). Since is presentable, we can choose an essentially small subcategory 0 such that the identity functor id is a left Kan extension of id 0. Let C 0 = C 0. Since the fibers of q are essentially small, the -category C 0 is essentially small. We will prove that q is a left Kan extension of q C 0. The existence of a colimit for q will then follow from Lemma T , and the equivalence q (C) lim(q ) lim (q) q(c) will follow from Theorem T (since we may assume without loss of generality that C C 0 ). Fix an object C C; we wish to prove that q exhibits q (C) as a colimit of the diagram C 0 C C /C C q. Let C denote the full subcategory of C 0 C C /C spanned by those objects which correspond to q-cartesian morphisms D C. The inclusion C C 0 C C /C admits a left adjoint and is therefore left cofinal. It will therefore suffice to show that q exhibits q (C) as a colimit of q C. Note that q induces an equivalence C 0 /q(c). The assumption on α guarantees that q C can be identified with the functor D q(d) q(c) q (C). Since colimits in are universal, it suffices to show that q(c) is the colimit of the forgetful functor 0 /q(c) ; this follows from our assumption that id is a left Kan extension of id 0. We now prove (2). We work by induction on the cardinality of I. Suppose first that I is empty; in this case we must prove that lim(q) is a final object of. We first show that q is left cofinal. Lemma T , we are reduced to proving that each fiber of q is weakly contractible. This follows from our assumption that the fibers of q are sifted (Proposition T ). Since q is left cofinal, we have an equivalence lim(q) lim(id ). Since has a final object 1, we conclude that lim(id ) 1 as desired. In the case where I has a single element, there is nothing to prove. If I has more than one element, we can write I as a disjoint union of proper subsets I and I +. Let q = i I q i and q + = i I + q i (the product taken in Fun(C, ) /q ), so that q q q q +. Using the inductive hypothesis, we are reduced to proving that lim(q ) lim(q ) lim(q + ). We next prove that the diagonal map δ : C C C is left cofinal. According to Theorem T , it will suffice to show that for every pair of objects (C, D) C having the same image U, the fiber product C = C C C(C C) (C,D)/ is weakly contractible. Let C denote the fiber product C {U}. Since q is a Cartesian fibration, the inclusion C C admits a right adjoint; it will therefore suffice to show that C is contractible. This follows from Theorem T , since the diagonal map C U C U C U is left cofinal by virtue of our assumption that C U is sifted. We define functors f, f, f + : C C by the formulas f (C, D) = q (C) f(c, D) = q (C) q(c) q + (D) f + (C, D) = q + (D). There are evident natural transformations f f f +. Note that q = f δ, q = f δ, and q + = f + δ. Since δ is left cofinal, we are reduced to proving that the canonical map lim(f) lim(f ) lim(f + ) is an equivalence. Let ɛ : C C C C denote a right adjoint to the inclusion C C C C. Concretely, ɛ is given by the formula (C, D) (π1c, π2d), where π 1 and π 2 are the projection maps q(c) q(d) q(c) and q(c) q(d) q(d). Since ɛ admits a left adjoint, it is left cofinal. We are therefore reduced to proving that the map lim(f ɛ) lim (f ɛ) lim(f + ɛ) is an equivalence. Since the natural transformations q q q + satisfy ( ), we see that these functors are given informally by the formulas (f ɛ)(c, D) = q (C) q(d) (f ɛ)(c, D) = q (C) q + (D) (f + ɛ)(c, D) = q(c) q + (D). Since colimits in are universal, we are reduced to proving that lim (q ) lim(q) lim(q ) lim(q + ) lim(q) lim(q + ) is a limit diagram in. This follows immediately from our observation that lim (q) is a final object of. 9

10 Remark 2.4. Let C be a nonempty -category such that every pair of objects of C admits a coproduct in C. Then C is sifted: this is an immediately consequence of Theorem T Proof of Proposition 2.2. Let E = C T 1, and let E 0 denote the full subcategory of E spanned by C T 1 and C T {1}. Let v denote the cone point of C. The functors q and Q together determine a map F : E. We first claim that there exists a functor F : E which is a left Kan extension of F. In view of Lemma T , it will suffice to prove that for every object U T, the diagram F (E 0 ) /(v,u,0) admits a colimit in. Let q denote the composition of this diagram with the left cofinal inclusion C {U} {0} (E 0 ) /(v,u,0) ; we are therefore reduced to proving that q has a colimit in. The restriction of F to C {U} 1 determines a natural transformation α : q q. Since Q carries q-cartesian morphisms in C to p-cartesian morphisms in Str loc T (), the natural transformation α satisfies condition ( ) of Lemma 2.3, so that q admits a colimit in. This proves the existence of F, which is in turn equivalent to giving a functor Q : C Fun(T, Fun( 1, )) Fun(T,Fun({1},)) satisfying the hypothesis of (3). We next claim: (a) For every admissible morphism U in T and every object C C, the diagram F (C, U, 0) F (v, U, 0) F (C,, 0) F (v,, 0) is a pullback square in. To prove (a), it will suffice (by Theorem T ) to show that for every morphism C D in C, the diagram F (C, U, 0) F (D, U, 0) F (C,, 0) F (D,, 0) is a pullback square in. This follows from our assumption that the map Q(C) Q(D) is a morphism in Str loc T (). We now claim that Q factors through Str T (): that is, that Q(v) is a T-structure on /1. We must verify that Q(v) : T satisfies the conditions listed in Definition V.3.1.4: (i) The functor Q(v) preserves finite products. Lemma 2.3. This follows immediately from the second assertion of (ii) Suppose we are given a pullback diagram U U in T, where the vertical maps are admissible. We wish to prove that the induced diagram F (v, U, 0) F (v, U, 0) F (v,, 0) F (v,, 0) 10

11 is a pullback square in. In view of Theorem T , it will suffice to show that for each C C, the outer square in the diagram F (C, U, 0) F (C, U, 0) F (v, U, 0) F (C,, 0) F (C,, 0) F (v,, 0) is a pullback. The square on the left is a pullback since Q(C) Str T (), and the square on the right is a pullback by (a). (iii) Suppose we are given an admissible covering {U i U} in T. We wish to prove that the induced map i F (v, U i, 0) F (v, U, 0) is an effective epimorphism. For this, it suffices to show that the composite map i,c F (C, U i, 0) F (v, U, 0) is an effective epimorphism. This map factors as a composition i,c F (C, U i, 0) C F (C, U, 0) F (v, U, 0) where the first map is an effective epimorphism (since each Q(C) belongs to Str T ()) and the second map is an effective epimorphism because F (v, U, 0) lim C F (C, U, 0). We next claim that Q factors through Str loc T (). In other words, we claim that for each C C, the map Q(C) Q(v) belongs to Str loc T (). This follows immediately from (a). To complete the proof of (1) and (2) and the if direction of (3), it will suffice to show that for every geometric morphism f : Y, the pullback f Q is a p -colimit diagram in Str loc T (Y), where p : Str loc T (Y) Y denotes the projection map. Note that p is the restriction of a map p : Str T (Y) Y, where Str T (Y) is a full subcategory of Fun(T, Fun( 1, Y)) Fun(T,Fun({1},Y)) Y. Since f preserves small colimits, the proof of Lemma 2.3 shows that f F is a left Kan extension of f F, so that f Q is a p -colimit diagram. We wish to show that this p -colimit diagram is also a p -colimit diagram: in other words, that if O Str T (Y), then a morphism f Q(v) O belongs to Str loc T (Y) if and only if each of the composite maps f Q(C) f Q(v) O belongs to Str loc T (Y). The only if direction is clear. Conversely, suppose that each composite map f Q(C) O belongs to Str loc T (Y), and let U be an admissible morphism in T. For each C C, the canonical map f F (C, U, 0) f F (C,, 0) O() O(U) is a pullback square. Since colimits in Y are universal, we conclude that the canonical map f F (v, U, 0) f F (v,, 0) O() O(U) is an equivalence, so that f Q(v) O belongs to Str loc T (Y) as desired. The only if direction of (3) now follows from the uniqueness of p-limit diagrams. Let us now suppose that T is an arbitrary pregeometry, and that O is a T-structure on an -topos. We would like to show that there exists a diagram Q : C Str loc T () satisfying the hypotheses of Proposition 2.2, where each Q(C) is a reasonably simple object of Str loc T (), and O is a colimit of the diagram Q relative to the projection Str loc T (). Our first goal is to formulate precisely what it means for the objects Q(C) to be reasonably simple. Notation 2.5. Let T be a pregeometry. We let Spec T : T Top(T) be the absolute spectrum functor of Remark V For every object U T, we will denote the T-structured -topos Spec T (U) by ( U, O U ). Definition 2.6. Let be an -topos, T a pregeometry, and O an object of Str loc T (). We will say that O is elementary if there exists an object T and a morphism f : (, O) Spec T () in Top(T) which induces an equivalence f O O. More generally, we say that an object (O, U) Str loc T () is elementary if O is elementary when viewed as an object of Str loc T ( /U ). 11

12 Remark 2.7. Let be an -topos, T a pregeometry, and fix an object O Str loc T () Str loc T ( /1 ) (where 1 denotes a final object of ). For each object T and every map η : U O() in, we can identify η with a global section of O() /U, which is classified by a map f : (U, O U) Spec T () in Top(T). We let O η denote the pullback f O Str loc T ( /U ). Note that f induces a map α : (U, O η ) (1, O) in Str loc T (), and that (U, O η ) is elementary. Unwinding the definitions, we see that a morphism α : (U, O ) (1, O) in Str loc T () arises via this construction (for some and some map η : U O()) if and only if (U, O ) is elementary. Note however that the object T and the morphism η : U O() are generally not determined by α. Remark 2.8. Let, T, and O be as in Remark 2.7. Choose an object T and a map η : U O() in, so that the object (U, O η ) Str loc T () is defined. By construction, the map η lifts canonically to a map η : U O η () (in /U ). Using the universal property of Spec T (), we obtain the following universal property O η : for every map O O U in Str loc T ( /U ), evaluation on η determines a homotopy fiber sequence Map Str loc T (/U ) / O U (O η, O ) Map /U (U, O ()) Map (U, O()) (where the fiber is taken over the point η Map (U, O())). Remark 2.9. Let, T, and O be as in Remark 2.7. Let T be a product of finitely many objects i T, and let η : U O() be a morphism in which determines maps η i : U O( i ). The universal property of Remark 2.8 shows that O η can be identified with the coproduct of the objects O ηi in the -category Str loc T ( /U ) / O U. Remark Let, T, and O be as in Remark 2.7. Let T be an object and let η : U O() be a morphism in. Suppose that f : V U is another morphism in, and let f : Str loc T ( /U ) Str loc T ( /V ) be the induced pullback map. Then there is a canonical equivalence f O η O η f in Str loc T ( /V ). We now formulate a precise sense in which the -category Str loc T () is locally generated by elementary objects under sifted colimits. Proposition Let be an -topos, T a pregeometry, and O an object of Str loc T (). Then there exists a Cartesian fibration q : C and a diagram Q : C Str loc () with the following properties: T (1) Let p : Str loc T () be the forgetful functor. The composition p Q is an extension of q which carries the cone point of C to a final object 1. (2) For each object C C, the object Q(C) Str loc T () is elementary. (3) The fibers of q are essentially small sifted -categories. (4) The functor Q carries q-cartesian morphisms in C to p-cartesian morphisms in Str loc T (). (5) The map Q is a p-colimit diagram (see Proposition 2.2). (6) The map Q carries the cone point of C to an object of Str loc T ( /1 ) Str loc T () equivalent to O. Proposition 2.11 is an immediate consequence of the following pair of lemmas: Lemma Let be an -topos, T a pregeometry, and O : T an object of Str loc T (), which we identify O with an object of Str loc T () lying over the final object 1 of. Let E Str loc T () /(1,O) be the full subcategory spanned by those morphisms (U, O ) (1, O) such that O is elementary. Then the forgetful functor q : E is a Cartesian fibration, and the fibers of q are essentially small -categories which admit finite coproducts (and are therefore sifted; see Remark 2.4). 12

13 Lemma Let be an -topos, T a pregeometry, and O : T an object of Str loc T (). Let p : () be the canonical map, and let E be defined as in Lemma Then the map Str loc T is a p-colimit diagram. E Str loc T () (1,O) Strloc () Remark More informally, Lemma 2.13 asserts that if O : T is a T-structure on, then O can be identified with the p-colimit of {(U, O )}, where (U, O ) ranges over all locally elementary objects of Str loc T () equipped with a map (U, O ) (1, O). Proof of Lemma It follows from Remark 2.10 that q is a Cartesian fibration, and Remark 2.9 guarantees that the fibers of q admit finite coproducts. It remains to show that for each object U, the fiber q 1 {U} essentially small. According to Remark 2.7, every object of q 1 {U} is given by the morphism (U, O η ) (1, O) determined by an object T and a morphism η : U O() in. Since T is essentially small and each mapping space Map (U, O()) is essentially small, we conclude that q 1 {U} is essentially small. Proof of Lemma Using Lemma 2.12 and the description of p-colimit diagrams given by Proposition 2.2, we are reduced to proving the following assertion, for every T: ( ) Let e : Str loc T () be the map given by evaluation at T and the vertex {1} 1. Then the canonical map E Str loc T () e is a colimit diagram. To prove ( ), consider the canonical functor E Fun( 1, ), which carries a morphism (U, O ) (1, O) to the map O () O() in. Let E = E Fun( {1,2},) Fun( 2, ). There is an evident projection map q : E E. This projection map admits a section s, which carries an object α : (U, O ) (1, O) of E to the pair (α, σ), where σ is the 2-simplex in. Let e denote the composite map O () O () T O() E Fun( 2, ) Fun({0}, ), so that e E e s. Consequently, to prove ( ), it will suffice to show that the canonical map lim(e s) O() is an equivalence in. We note that the identity transformation p s id E is the counit of an adjunction between p and s. In particular, s is left cofinal; it will therefore suffice to show that the canonical map lim(e ) O() is an equivalence in. Note that the inclusion {0,2} 2 determines a factorization of e as a composition E e 0 / O() e 0. The functor e 0 admits a left adjoint, which carries a morphism η : U O() to the pair (α, σ) E, where α : (U, O η ) (1, O) is as in Remark 2.7 and σ is the diagram U O η () η O(). It follows that e 0 is left cofinal. We are therefore reduced to showing that the canonical map lim(e 0) O() is an equivalence. This is clear, since / O() contains O() as a final object. 13

14 3 The Proof of Proposition 1.8 Our goal in this section is to give a proof of Proposition 1.8, which was stated in 1. The basic idea is to use the results of 2 to reduce the proof to an easy special case: Lemma 3.1. Let be an -topos and T an unramified pregeometry. Suppose we are given a morphism f : Y in T and another object Z T. Let g,z : Z be a geometric morphism inducing further geometric morphisms Then: g,y,z : Y Z g : g,y : Y. (1) The diagram σ : g,y O Y g O g,y,z O Y Z β g,z O Z is a pushout square in Str loc T (). (2) For every geometric morphism g : Y, the image g (σ) is a pushout square in Str loc T (Y). (3) The map β is an effective epimorphism. Proof. Let p denote the forgetful functor Top(T) Top(T), and let τ denote the commutative diagram Spec T ( Z) Spec T () Spec T ( Y Z) Spec T ( Y ) in L Top(T) op. Then τ is a pullback square. The assumption that T is unramified guarantees that p(τ) is also a pullback square. It follows from Proposition T that τ is a p-limit diagram. Assertions (1) and (2) now follow from Propositions T and T Assertion (3) is obvious, since β admits a section (induced by the projection map Y Z Z in T). Our strategy now is to reduce the general case of Proposition 1.8 to the special case treated in Lemma 3.1 by choosing a suitable resolution of the diagram O 0 β O α O. To accomplish this, we will prove a slightly more complicated version of Proposition First, we need some terminology. Definition 3.2. Let T be a pregeometry and an -topos. Suppose we are given a diagram τ : O 0 β O α O in Str loc T (). We will say that τ is elementary if there exists a morphism f : Y in T, an object Z T, and a geometric morphism g,z : Z such that α can be identified with the induced map g,y O Y g O and β with the map g,y O Y g,y,z O Y Z, where g and g,y,z are defined as in the statement of Lemma

15 β α Remark 3.3. If O 0 O O is an elementary diagram in Str loc T (), then O 0, O, and O are elementary objects of Str loc T (). Conversely, if O is an elementary object of Str loc T (), then the constant diagram O id O id O, is elementary. Remark 3.4. Any morphism α : O O in Str loc T () which admits a section is an effective epimorphism. β α In particular, if we are given an elementary diagram O 0 O O in Str loc T (), then α is an effective epimorphism. We will need the following converse of Remark 3.4: Proposition 3.5. Let be an -topos, T a pregeometry, and τ : Λ 2 0 Str loc T ( /1 ) Str loc T () a diagram (here 1 denotes the final object of ), which we depict as O 0 β O α O. Assume that α is an effective epimorphism. Then there exists a Cartesian fibration q : C and a diagram Q : C Λ 2 0 Str loc T () with the following properties: (1) Let p : Str loc () be the forgetful functor. Then there is a commutative diagram T C Λ 2 0 C Q q Str loc T (), p where q C = q and q carries the cone point of C to 1. (2) For each object C C, restriction of Q to {C} Λ 2 0 determines an elementary diagram in Str loc T ( /q(c) ). (3) The fibers of q are essentially small sifted -categories (in fact, we can assume that the fibers of q are nonempty and admit coproducts for pairs of objects; see Remark 2.4). (4) For each vertex i Λ 2 0, the restriction Q C {i} carries q-cartesian morphisms in C to p-cartesian morphisms in Str loc T (). (5) For each vertex i Λ 2 0, the restriction Q C {i} is a p-colimit diagram in Str loc T (). (6) The map Q carries the cone point of C to the diagram τ. Remark 3.6. Proposition 2.11 is an immediate consequence of Proposition 3.5, applied to the case where τ is a constant diagram; see Remark 3.3. Assuming Proposition 3.5 for the moment, we can complete the proof of our main result. Proof of Proposition 1.8. Assume that T is unramified and that we are given a map τ : Λ 2 0 Str loc T ( /1 ), corresponding to a diagram O 0 O α O where α is an effective epimorphism. Choose a diagram C Λ 2 0 C Q q Str loc T () p 15

16 satisfying the conclusions of Proposition 3.5. Let v denote the cone point of C, q = q C, and let D denote the full subcategory of C 1 1 obtained by omitting the final object. We first claim that Q admits a p-left Kan extension Q : D Str loc T () (compatible with q). In view of Lemma T , it will suffice to show that for every object C C, the restriction Q (C /C Λ 2 0) admits a p-colimit compatible with q. Since the inclusion {id C } C /C is left cofinal and p is a Cartesian fibration, it suffices to show that Q {C} Λ 2 0 admits a colimit in Str loc T ( /q(c) ). Since Q {C} Λ 2 0 is an elementary diagram and T is unramified, this follows from Lemma 3.1. Moreover, we learn that Q 0 = Q C {(1, 1)} carries q-cartesian morphisms of C to p-cartesian morphisms of Str loc T (). Furthermore, for every geometric morphism f : Y, the composition f Q is a p -left Kan extension of f Q, where p denotes the projection Str loc T (Y) Y. We next claim that Q can be extended to a p-colimit diagram Q : C 1 1 Str loc T () (compatible with q). Let Q 1 = Q (C 1 1 ). Our assumptions guarantee that Q is a p-left Kan extension of Q 1. Using Lemma T , it suffices to show that Q 1 can be extended to a p-colimit diagram (compatible with q). Since the inclusion C {(1, 1)} D is left cofinal, it suffices to show that Q 0 can be extended to a p-colimit diagram compatible with q, which follows immediately from Proposition 2.2. Using Proposition T , we conclude that Q is a p-left Kan extension of Q. Moreover, the same arguments show that f Q is a p -left Kan extension of f Q, for every geometric morphism f : Y. Let Q = Q (C Λ 2 0). For every geometric morphism f : Y, f Q is a p -colimit diagram. Since the inclusion {v} Λ 2 0 C Λ 2 0, it follows that f (Q {v} 1 1 ) is p -colimit diagram in Str loc T (Y). In particular, setting O 0 = Q (v, 1, 1), we deduce that the diagram f O f O f O 0 f O 0 is a pushout square in Str loc T (Y). This completes the proofs of (1) and (3). To complete the proof, we must show that the map O O 0 is an effective epimorphism of T-structures on. Fix T; we wish to show that the morphism u : Q (v, 0, 1)() Q (v, 1, 1)() is an effective epimorphism in. We note that u is the colimit of a diagram of morphisms u C : Q (C, 0, 1)() Q (C, 1, 1)() indexed by C C. It therefore suffices to show that each u C is an effective epimorphism. This is clear, since u C admits a section (by virtue of our assumption that Q ({C} Λ 2 0) is elementary) The remainder of this section is devoted to the proof of Proposition 3.5. Construction 3.7. Let NFin denote the category of nonempty finite sets, and let NFin correspond to a set with one element. Fix a pregeometry T, an -topos containing a final object 1, and an object O Str loc T (). Let p : Str loc T () /(1,O) be the projection map and let E Str loc T () /(1,O) be as in Lemma Note that p is a Cartesian fibration and restricts to a Cartesian fibration p 0 : E. It follows from Remark 2.9 that for every object U, the fiber E U = p 1 0 {U} admits finite coproducts (which are also coproducts in the larger -category p 1 {U}). Since p 0 is a Cartesian fibration, Corollary T implies that finite coproducts in E U are also p 0 -coproduct diagrams (and even p-coproduct diagrams). Let v : E N(NFin) E be the projection onto the first factor. Using Lemma T , we deduce that there exists another functor u : E N(NFin) E such that p 0 u = p 0 v, such that u (E { }) is the identity map and u is a p-left Kan extension of u (E { }). Since u (E { }) = v (E { }), there is an essentially unique natural transformation γ : u v such that p(α) = id and γ restricts to the identity on E {[0]}. Let α : (1, O) (1, O) be a morphism in Str loc T (). We let E denote the fiber product (E N(NFin)) Str loc T () Strloc /(1,O) T () /α, where E N( ) maps to Str loc T () /(1,O) via u. Amalgamating the projection map E Str loc T () /α with γ, we obtain a functor γ : E Fun( 1, Str loc T ()) /α. 16

17 Remark 3.8. In the situation of Construction 3.7, the forgetful functor p : E factors as a composition E p E N(NFin) p E p. The map p is a right fibration, the map p obviously a Cartesian fibration, and the map p is a Cartesian fibration by Lemma 2.12; it follows that p is a Cartesian fibration. It is easy to see that the fibers of p are nonempty (for each U, we can choose an object of E corresponding to a map η : U O(), where is a final object of T; for every finite set S, the object (U, O η, S) lifts to E in an essentially unique way). Since NFin admits finite coproducts and p is a right fibration, Proposition T and Lemma 2.12 guarantee that the fibers of p admit finite coproducts. It follows from Remark 2.4 that the fibers of p are sifted. Lemma 3.9. Let be an -topos, T a pregeometry, and p : Str loc T () the forgetful functor. Let q : K Str loc T () be a diagram such that q carries every morphism in K to a p-cartesian morphism in (), and p q is a colimit diagram in. Then q is a p-colimit diagram. Str loc T Proof. In view of Lemma VII.5.17, it will suffice to show that the Cartesian fibration p is classified by a functor χ : op Ĉat which preserves small limits. This is an easy consequence of Theorem T Lemma Let T be a pregeometry and an -topos, let α : O O be a morphism in Str loc T (), and let E and γ : E Fun( 1, Str loc ()) /α be as in Construction 3.7. Let p : Str loc Str loc T the projection map. We can identify γ with a map γ : E 1 Str loc T Then: T T ( /1 ) () be (). Assume that α is an effective epimorphism. ( ) For i {0, 1}, the restriction γ (E {i}) determines a p-colimit diagram in Str loc T (). Proof. We first prove that γ 0 is a p-colimit diagram. Let E E denote the inverse image of E { } in E. Note that γ 0 E is a p-left Kan extension of γ 0 E. Using Lemma T , we are reduced to proving that γ 0 E is a p-colimit diagram. Note γ 0 induces a fully faithful embedding E Str loc T () /(1,O), whose essential image is the collection of maps (U, O ) (1, O) where O is elementary. The desired result now follows from Lemma We now prove that γ 1 is a p-colimit diagram. Let E 0 denote the essential image of the forgetful functor E E. More concretely, an object β : (U, O ) (1, O) of E belongs to E 0 if β can be lifted to a commutative diagram (U, O ) We observe that γ 1 factors as a composition We will prove: (a) The forgetful functor E E 0 is left cofinal. β (1, O) β α E δ E δ 0 Str T (). (b) The diagram δ E is a p-left Kan extension of δ E 0. (1, O). Note that δ is a p-colimit diagram (Lemma 2.13); combining this with (b) and Lemma T , we conclude that δ E 0 is a p-colimit diagram. Using (a) we conclude that δ δ = γ 1 is a p-colimit diagram as desired. It remains to prove (a) and (b). We first prove (a). We have seen above that the forgetful functor E E is a Cartesian fibration (so that E 0 E is a sieve). According to Lemma T , it will suffice to show 17

18 that for each object E E 0, the fiber E E {E} is weakly contractible. This follows from Remark 2.4 and Proposition T , since E E {E} is a nonempty -category which admits pairwise coproducts. We now prove (b). Fix an object E E, corresponding to a map (U, O ) (1, O) in Str loc T (), and let (E 0 ) /E denote the fiber product E 0 E E /E. We wish to prove that δ exhibits δ(e) = (U, O ) as a p-colimit of the diagram δ (E 0 ) /E. Since α is an effective epimorphism, there exists an effective epimorphism f : V U such that the composite map (V, O V ) (U, O ) (1, O) factors through (1, O). Let D E /E denote the full subcategory of E /E spanned by those objects which determine maps (Ũ, Õ ) (U, O ) such that Ũ U factors through f. Note that D (E 0 ) /E. We will prove: (c) The forgetful functor θ : E /E E Str loc T () is a p-left Kan extension of its restriction to D. Assertion (c) implies that E is a p-colimit of the diagram θ D, so that E is a p-colimit of θ (E 0 ) /E by Lemma T and the proof is complete. To prove (c), choose an object Ẽ E /E, corresponding to a map (Ũ, Õ) (U, O) in Strloc T (). We wish to prove that θ exhibits Ẽ as a p-colimit of θ (D E /E E / Ẽ ). Replacing E by Ẽ (and V by V U Ũ), we are reduced to proving that θ exhibits E as a colimit of θ D. Let V be the Čech nerve of the map f : V U, and let O be the pullback of O to V. The construction [n] (V n, O n) determines a left cofinal map N( ) op D; it will therefore suffice to show that θ exhibits E = (U, O ) as a p-colimit of the simplicial object (V, O ) of Str loc T (). Since f is an effective epimorphism, we have U V and the desired result follows from Lemma 3.9. Proof of Proposition 3.5. Fix an -topos containing a final object 1, a pregeometry T, and a diagram O 0 β O α O in Str loc T ( /1 ) Str loc T (). Let E be defined as in Construction 3.7. Note that evaluation at {0} 1 induces a functor E Str loc T. We let C denote the full subcategory of the fiber product () /(1,O) E Fun ( {0},Str loc T () /(1,O)) Fun ( 1, Str loc T () /β) spanned by those objects whose image in Fun ( 1, Str loc T () /β) corresponds to a commutative diagram (U, O ) (U, O 0) (1, O) β (1, O 0 ) with the following property: there exists an object T and a morphism η : U O 0 () in such that O 0 is a coproduct of O with O η in Str loc T ( /U ). By construction, the maps C E /O0 U Fun( 1, Str loc T ()) /α and C Fun( 1, Str loc T ()) /β amalgamate to determine a map C Λ 2 0 Str loc T () satisfying conditions (1), (2), (3), (4), and (6). It remains to verify (5). Let D be the full subcategory of Str loc T () /(1,O 0) spanned by those morphisms (U, O 0) (1, O 0 ) such that (U, O 0) is elementary. We have an evident pair of forgetful functors D u C v E. In view of Lemmas 3.10 and 2.13, it will suffice to show that the maps u and v are left cofinal. The map v is left cofinal because it has a right adjoint: in fact, v has a fully faithful right adjoint, whose essential image is the full subcategory of C spanned by those objects whose image in Fun ( 1, Str loc T () /β ) 18

19 corresponds to a commutative diagram (U, O ) β (U, O 0) (1, O) β (1, O 0 ) such that β is an equivalence. We will prove that u is left cofinal using Theorem T Fix an object of D D, corresponding to a map (U, O 0) (1, O 0 ). We wish to prove that the -category C D/ = C D D D/ is weakly contractible. Let C be the full subcategory of C D/ spanned by those objects C whose images in D D/ and E Str loc T () /(1,O). It is not difficult to show that the inclusion C C D/ admits a right adjoint, and is therefore a weak homotopy equivalence. We now observe that the forgetful functor C N(NFin) is a trivial Kan fibration, so that C is sifted (Remark 2.4) and therefore weakly contractible (Proposition T ). 4 Closed Immersions in Spectral Algebraic Geometry In 1, we introduced the notion of an unramified pregeometry. Roughly speaking, a pregeometry T is unramified if the there is a good theory of closed immersions between T-structured -topoi. Our goal in this section is to give some examples of unramified pregeometries which arise in the study of spectral algebraic geometry. Proposition 4.1. Let k be a connective E -ring. Then the pregeometry T Sp Zar (k) of Definition VII.2.18 and the pregeometry and T Sp ét (k) of Definition VII.8.26 are unramified. Proof. We will give the proof for T Sp ét (k); the proof for the pregeometry TSp Zar (k) is similar (and easier). Unwinding the definitions, we are reduced to proving the following assertion: ( ) Let A, B, and C be E -algebras over k which belong to T Sp ét (k), and let φ : B A be a morphism in CAlg k. In the diagram A k B A A k B k C in CAlg k determines a pullback square of -topoi Shv((CAlgét A k B) op ) ψ A k C Shv((CAlgét A) op ) Shv((CAlgét A k B k C) op ) Shv((CAlgét A k C) op ) To prove ( ), let U = Shv((CAlgét A k B) op ) be defined by the formula { 0 if R A k B A 0 U(R) = otherwise. 19

20 and let U denote the inverse image of U in = Shv((CAlgét A k B k C) op ). The proof of Proposition VIII allows us to identify σ with the diagram of -topoi /U /U, which is pullback square by Proposition T Remark 4.2. Let k be a connective E -ring and let T be one of pregeometries T Sp disc (k), TSp Zar (k), or TSp ét (k). Let be an -topos, and let α : O O be a local morphism of sheaves of T-structures on. The following conditions are equivalent: (1) The map α is an effective epimorphism. (2) The map α induces an effective epimorphism O(k{x}) O (k{x}), where k{x} denotes the free E - algebra over k on one generator. The implication (1) (2) is obvious, and the implication (2) (1) follows from the observation that every object U T admits an admissible morphism U, where is a product of finitely many copies of k{x}. Remark 4.3. Let f : (, O ) (Y, O Y ) be a morphism of spectrally ringed -topoi. Assume that O and O Y are connective, so that we can identify O and O Y with T Sp disc-structures on and Y, respectively. Then f is a closed immersion in Top(T Sp disc ) if and only if the following conditions are satisfied: (a) The underlying geometric morphism of -topoi f : Y is a closed immersion. (b) The map of structure sheaves f O Y O is is connective: that is, fib(f O Y O ) is a connective sheaf of spectra on. If the morphism f belongs to the subcategory RingTop Zar RingTop, then f is a closed immersion in Top(T Sp Zar ) if and only if conditions (a) and (b) are satisfied. If f belongs to the subcategory RingTop ét RingTop Zar, then f is closed immersion in Top(T Sp ét ) if and only (a) and (b) are satisfied. It follows from Propositions 4.1 that there is a good theory of closed immersions in the setting of spectral algebraic geometry. Our other goal in this section is to give an explicit description of the local structure of a closed immersion. To simplify the exposition, we will restrict our attention to the case of spectral Deligne-Mumford stacks. Theorem 4.4. Let A be a connective E -ring and let f : (, O) Spec A be a map of spectral Deligne- Mumford stacks. Then f is a closed immersion if and only if there exists an equivalence (, O) Spec B for which the induced map A B induces a surjection π 0 A π 0 B. Remark 4.5. Theorem 4.4 has an evident analogue in the setting of spectral schemes, which can be proven by the same methods. Remark 4.6. The condition that a morphism f : Y of spectral Deligne-Mumford stacks be a closed immersion is local on the target with respect to the étale topology (see Definition VIII.3.1.1). Together with Theorem 4.4, this observation completely determines the class of closed immersions between spectral Deligne-Mumford stacks. The proof of Theorem 4.4 will require a number of preliminaries. Lemma 4.7. Let be an -topos and let f : Y be a closed immersion of -topoi. If is n-coherent (for some n 0), then Y is n-coherent. 20

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