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Introductory Topics in Derived Algebraic Geometry

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Introductory Topics in Derived Algebraic Geometry Introductory topics in derived algebraic geometry Tony Pantev Gabriele Vezzosi Department of Mathematics DIMAI University of Pennsylvania (US) Università di Firenze (Italy) February 2018 Abstract We give a quick introduction to derived algebraic geometry (DAG) sampling basic constructions and techniques. We discuss affine derived schemes, derived algebraic stacks, and the Artin-Lurie representability theorem. Through the example of deformations of smooth and proper schemes, we explain how DAG sheds light on classical deformation theory. In the last two sections, we introduce differential forms on derived stacks, and then specialize to shifted symplectic forms, giving the main existence theorems proved in [PTVV]. Contents 1 Introduction 1 2 Affine derived geometry 2 3 Étale topology and derived stacks 9 4 Derived algebraic stacks 11 5 Lurie’s Representability Theorem 13 6 DAG “explains” classical deformation theory 15 7 Forms and closed forms 20 8 Shifted symplectic geometry 27 1 Introduction Derived Algebraic Geometry (DAG) starts with the idea of replacing the affine objects of Algebraic Geometry, i.e. commutative rings, by some kind of “derived commutative rings” whose internal homotopy theory is non trivial. This can be achieved over Q by considering commutative differential non-positively graded algebras (cdga’s), while in general one might instead consider simplicial commutative algebras.1 For simplicity, we will stick to the case of cdga’s (i.e. we will assume to work over Q). As in classical 1 Note that DAG based on cdga’s over Q or on simplicial commutative Q-algebras are equivalent theories. 1 Algebraic Geometry, the first step is to develop the local or affine theory, i.e to define and study finiteness conditions, flat, smooth, étale properties for morphisms between cdga’s. This is the content of Section 2. Once this is set up, we will define the analog in DAG of being a sheaf or stack with respect to a derived version of a topology on these derived affine objects (Section 3), and then specify which sheaves or stacks are of geometric type (these will be called derived algebraic or derived Artin stacks). This last step will be done by using atlases and a recursion on the “level of algebraicity” in Section 4 where we will also list the main basic properties of derived algebraic stacks. It’s not always easy to decide whether a given derived stack is algebraic by using only the definition, and a powerful criterion is given by J. Lurie’s DAG version of M. Artin’s representability theorem: this is explained in Section 5, where we also give a simplified but often very useful version of Lurie’s representability. In Section 6 we explain, through the example of deformations of a given smooth proper scheme, how DAG fills some conceptual gaps in classical deformation theory, the idea being that once we allow ourselves to consider also deformations over an affine derived base, then deformation theory becomes completely transparent. The second part of this article describes symplectic geometry in DAG. We start by describing differential forms and closed differential forms on a derived stack (Section 7). These forms have two new features with respect to the classical case: first of all they have a degree (this new degree of freedom comes from the fact that in DAG the module of K´’ahler differentials is replaced by a complex, the so-called cotangent complex), secondly the notion of a closed form in DAG consists of a datum rather than a property. Once these notions are in place, the definition of a derived version of symplectic structure is easy. The final Section 8 reviews some the main examples and existence results in the theory of derived symplectic geometry taken from [PTVV], namely the derived symplectic structure on the mapping derived stack of maps from a O-compact oriented derived stack to a symplectic derived stack, and the derived symplectic structure on a lagrangian intersection. 2 Affine derived geometry 2.1 Homotopical algebra of dg-modules and cdga in characteristic 0 Let k be a commutative Q-algebra. Definition 2.1 We will write ¤0 • dgmodk for the model category of non-positively graded dg-modules over k (with differential in- creasing the degree), with weak equaivalences W = “quasi-isomorphisms” and fibrations F ib= “surjections in deg ă 0 ”, endowed with the usual symmetric b structure compatible with the model structure (i.e. a symmetric monoidal model category, [Hov]). ¤0 • dgmodk for the symmetric monoidal 8-category obtained by inverting (in the 8-categorical 2 ¤0 sense ) quasi-isomorphisms in dgmodk . ¤0 ¤0 • cdgak :“ CAlgpdgmodk q for the model category of commutative unital monoids in ¤0 dgmodk , with W = “quasi-isomorphisms” and F ib= “surjections in deg ă 0 ” (here we need that k is a Q-algebra). These are non-positively graded commutative differential graded k-algebras (cdga’s for short). 2See M. Robalo’s paper in this volume. 2 ¤0 ¤0 • cdgak pkq :“ CAlgpdgmodk q for the 8-category of non-positively graded commutative differ- ential graded k-algebras (derived k-algebras) that can also be obtained by inverting (in the 8- ¤0 categorical sense) quasi-isomorphisms in cdgak . • S will denote the 8-category of spaces. We will use analogous notations for unbounded dg-modules over k, and unbounded cdga’s over k, by simply omitting the p´q¤0 suffix. Same convention for non necessarily commutative algebras (by writing Alg instead of CAlg). There is an 8-adjunction: Symk dgmod¤0 / cdga¤0 k o k U (induced by a Quillen adjunction on the corresponding model categories) where Symk denotes the free cdga functor, and U the forgetful functor. ¤0 ‚ Mapping spaces. We have the following explicit models for mapping spaces in cdgak . Let Ωn be ‚ the algebraic de Rham complex of krt0; t1; ¨ ¨ ¨ ; tns{p i ti ´ 1q over k. Consider rns ÞÑ Ωn as a simplicial ¤0 ‚ object in cdgak. Thus, if B P cdgak , then the assignment rns ÞÑ τ¤0pΩn bk Bq defines a simplicial ¤0 ř object in cdgak . ¤0 For any pair pA; Bq in cdgak , there is an equivalence of of spaces (simplicial sets) ‚ Map ¤0 pA; Bq » prns ÞÑ Hom ¤0 pQkA; τ¤0pΩn bk Bqqq cdgak cdgak ¤0 where QkA is a cofibrant replacement of A in the model category cdgak . Cautionary exercises. The following exercises are meant to make the reader aware of the boundaries of the territory where we will be working. Exercise 2.2 Let charpkq “ p. Show that CAlgpdgmodkq cannot have a model structure with W=“quasi- isomorphisms” and Fib=“ degreewise surjections”. i Hint: 1) Construct a map f : A Ñ B in CAlgpdgmodkq with the following property: Di ; α P H pBq such that αp is not in the image of Hippfq. 2) Prove that no such f can be factored as F ib ˝ pW X Cofq. Exercise 2.3 Let charpkq “ 0. Show that the obvious 8-functor CAlgpdgmodkq Ñ Algpdgmodkq, though conservative, is not fully faithful. ¤0 For A P cdgak we consider dgmodpAq the symmetric monoidal 8-category of (unbounded) dg- modules over A, and we define ¤0 CAlgpdgmodpAqq » A{cdgak : 3 ¤0 Given any map f : A Ñ B P cdgak , there is an induced 8-adjunction ˚ f “p´qbAB dgmod B /dgmod A p qo p q f˚ ¤0 which is an equivalence of 8-categories if f is an equivalence in cdgak . 2.2 Cotangent complex ¤0 ¤0 ¤0 Let f : A Ñ B be a map in cdgak . For any M P dgmod pBq, let B ` M P cdgak be the trivial square zero extension of B by M (i.e. B acts on itself and on M in the obvious way, and M ¨ M “ 0). B ` M is naturally an A-algebra and it has a natural projection map prB : B ` M Ñ B of A-algebras. Definition 2.4 The space of derivations from B to M over A is defined as DerApB; Mq :“ Map ¤0 pB; B ` Mq: A{cdgak {B Equivalently, prB;˚ DerApB; Mq “ fib Map ¤0 pB; B ` Mq /Map ¤0 pB; Bq ; idB : A{cdgak A{cdgak ˆ ˙ ¤0 M Ñ DerApB; Mq can be constructed as an 8 functor dgmod pBq Ñ S, and we have the following derived analog of the classical existence results for Kähler differentials. ¤0 Proposition 2.5 The 8-functor M ÞÑ DerApB; Mq is corepresentable, i.e. D LB{A P dgmod pBq and a canonical equivalence DerApB; ´q » MapdgmodpBqpLB{A; ´q. ¤0 1 Proof. Let QAB » B be a cofibrant replacement of B in A{cdga . Then :“ Ω bQ B B k LB{A QAB{A A does the job. l Let us list the most useful properties of the cotangent complex construction (for details, see [HAG-II]). ¤0 (1) Given a commutative square in cdgak u1 A / A1 1 B u / B there is an induced map 1 LB{A Ñ u˚LB1{A1 ô LB{A bB B Ñ LB1{A1 : 3 In particular, we get a canonical map LB{A Ñ LH0pBq{H0pAq. If B “ S; A “ R are discrete , the 0 1 canonical map H pLS{Rq Ñ ΩS{R is an isomorphism. 3 ¤0 0 An object D P cdgak is discrete if the canonical map D Ñ H pDq is a quasi-isomorphism. 4 1 (2) If the commutative diagram in (1) is a pushout square, then the map LB{A bB B Ñ LB1{A1 is an equivalence. ¤0 (3) If C Ñ A Ñ B are maps in cdgak , then there is an induced cofiber sequence LA{C bA B Ñ LB{C Ñ LB{A 1 in dgmodpBq, where the first map is as in (1) via u “ idC , u : A Ñ B, and the second map is as 1 in (1) with u “ idB and u “ pC Ñ Aq).
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