FORMAL SCHEMES and FORMAL GROUPS Contents 1. Introduction 2
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Problems About Formal Groups and Cohomology Theories Arizona Winter School 2019
PROBLEMS ABOUT FORMAL GROUPS AND COHOMOLOGY THEORIES ARIZONA WINTER SCHOOL 2019 VESNA STOJANOSKA 1. Formal Group Laws In this document, a formal group law over a commutative ring R is always com- mutative and one-dimensional. Specifically, it is a power series F (x; y) 2 R[[x; y]] satisfying the following axioms: • F (x; 0) = x = F (0; x), • F (x; y) = F (y; x), and • F (x; F (y; z)) = F (F (x; y); z). Sometimes we denote F (x; y) by x +F y. For a non-negative integer n, the n-series of F is defined inductively by [0]F x = 0, and [n + 1]F x = x +F [n]F x. (1) Show that for any formal group law F (x; y) 2 R[[x; y]], x has a formal inverse. In other words, there is a power series i(x) 2 R[[x]] such that F (x; i(x)) = 0. As a consequence, we can define [−n]F x = i([n]F x). (2) Check that the following define formal group laws: (a) F (x; y) = x + y + uxy, where u is any unit in R. Also show that this is isomorphic to the multiplicative formal group law over R. x+y (b) F (x; y) = 1+xy . Hint: Note that if x = tanh(u) and y = tanh(v), then F (x; y) = tanh(x + y). (3) Suppose F; H are formal group laws over a complete local ring R, and we're given a morphism f : F ! H, i.e. f(x) 2 R[[x]] such that f(F (x; y)) = H(f(x); f(y)): Now let C be the category of complete local R-algebras and continuous maps between them; for T 2 C, denote by Spf(T ): C!Set the functor S 7! HomC(T;S). -
Formal Groups, Witt Vectors and Free Probability
Formal Groups, Witt vectors and Free Probability Roland Friedrich and John McKay December 6, 2019 Abstract We establish a link between free probability theory and Witt vectors, via the theory of formal groups. We derive an exponential isomorphism which expresses Voiculescu's free multiplicative convolution as a function of the free additive convolution . Subsequently we continue our previous discussion of the relation between complex cobordism and free probability. We show that the generic nth free cumulant corresponds to the cobordism class of the (n 1)-dimensional complex projective space. This permits us to relate several probability distributions− from random matrix theory to known genera, and to build a dictionary. Finally, we discuss aspects of free probability and the asymptotic representation theory of the symmetric group from a conformal field theoretic perspective and show that every distribution with mean zero is embeddable into the Universal Grassmannian of Sato-Segal-Wilson. MSC 2010: 46L54, 55N22, 57R77, Keywords: Free probability and free operator algebras, formal group laws, Witt vectors, complex cobordism (U- and SU-cobordism), non-crossing partitions, conformal field theory. Contents 1 Introduction 2 2 Free probability and formal group laws4 2.1 Generalised free probability . .4 arXiv:1204.6522v3 [math.OA] 5 Dec 2019 2.2 The Lie group Aut( )....................................5 O 2.3 Formal group laws . .6 2.4 The R- and S-transform . .7 2.5 Free cumulants . 11 3 Witt vectors and Hopf algebras 12 3.1 The affine group schemes . 12 3.2 Witt vectors, λ-rings and necklace algebras . 15 3.3 The induced ring structure . -
Formal Group Rings of Toric Varieties 3
FORMAL GROUP RINGS OF TORIC VARIETIES WANSHUN WONG Abstract. In this paper we use formal group rings to construct an alge- braic model of the T -equivariant oriented cohomology of smooth toric vari- eties. Then we compare our algebraic model with known results of equivariant cohomology of toric varieties to justify our construction. Finally we construct the algebraic counterpart of the pull-back and push-forward homomorphisms of blow-ups. 1. Introduction Let h be an algebraic oriented cohomology theory in the sense of Levine-Morel [14], where examples include the Chow group of algebraic cycles modulo rational equivalence, algebraic K-theory, connective K-theory, elliptic cohomology, and a universal such theory called algebraic cobordism. It is known that to any h one can associate a one-dimensional commutative formal group law F over the coefficient ring R = h(pt), given by c1(L1 ⊗ L2)= F (c1(L1),c1(L2)) for any line bundles L1, L2 on a smooth variety X, where c1 is the first Chern class. Let T be a split algebraic torus which acts on a smooth variety X. An object of interest is the T -equivariant cohomology ring hT (X) of X, and we would like to build an algebraic model for it. More precisely, instead of using geometric methods to compute hT (X), we want to use algebraic methods to construct another ring A (our algebraic model), such that A should be easy to compute and it gives infor- mation about hT (X). Of course the best case is that A and hT (X) are isomorphic. -
Congruences Between the Coefficients of the Tate Curve Via Formal Groups
PROCEEDINGS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 128, Number 3, Pages 677{681 S 0002-9939(99)05133-3 Article electronically published on July 6, 1999 CONGRUENCES BETWEEN THE COEFFICIENTS OF THE TATE CURVE VIA FORMAL GROUPS ANTONIOS BROUMAS (Communicated by David E. Rohrlich) 2 3 Abstract. Let Eq : Y + XY = X + h4X + h6 be the Tate curve with canonical differential, ! = dX=(2Y + X). If the characteristic is p>0, then the Hasse invariant, H, of the pair (Eq;!) should equal one. If p>3, then calculation of H leads to a nontrivial separable relation between the coefficients h4 and h6.Ifp=2orp= 3, Thakur related h4 and h6 via elementary methods and an identity of Ramanujan. Here, we treat uniformly all characteristics via explicit calculation of the formal group law of Eq. Our analysis was motivated by the study of the invariant A which is an infinite Witt vector generalizing the Hasse invariant. 1. Introduction The Tate curve in characteristic either zero or positive is an elliptic curve with canonical Weierstrass model: 2 3 (1) Eq : Y + XY = X + h4X + h6 5S +7S nk where h = 5S and h = 3 5 for S = : 4 − 3 6 − 12 k 1 qn n 1 X≥ − Note that the denominator appearing in the expression of h6 is simply a nota- tional convenience and in fact Eq is defined over Z[[q]]. Hence, since all coefficients are integral, we can specialize to positive characteristic p for any prime p of our choice and obtain Eq defined over Fp[[q]]. In all positive characteristics the Hasse invariant of the pair (Eq;!)for!the canonical invariant differential, ! = dX=(2Y + X), is equal to 1. -
Quasi-Coherent Sheaves on the Moduli Stack of Formal Groups
Quasi-coherent sheaves on the Moduli Stack of Formal Groups Paul G. Goerss∗ Abstract The central aim of this monograph is to provide decomposition results for quasi-coherent sheaves on the moduli stack of formal groups. These results will be based on the geometry of the stack itself, particularly the height filtration and an analysis of the formal neighborhoods of the ge- ometric points. The main theorems are algebraic chromatic convergence results and fracture square decompositions. There is a major technical hurdle in this story, as the moduli stack of formal groups does not have the finitness properties required of an algebraic stack as usually defined. This is not a conceptual problem, but in order to be clear on this point and to write down a self-contained narrative, I have included a great deal of discussion of the geometry of the stack itself, giving various equivalent descriptions. For years I have been echoing my betters, especially Mike Hopkins, and telling anyone who would listen that the chromatic picture of stable homotopy theory is dictated and controlled by the geometry of the moduli stack Mfg of smooth, one-dimensional formal groups. Specifically, I would say that the height filtration of Mfg dictates a canonical and natural decomposition of a quasi-coherent sheaf on Mfg, and this decomposition predicts and controls the chromatic decomposition of a finite spectrum. This sounds well, and is even true, but there is no single place in the literature where I could send anyone in order for him or her to get a clear, detailed, unified, and linear rendition of this story. -
FORMAL ALGEBRAIC SPACES 0AHW Contents 1. Introduction 1 2
FORMAL ALGEBRAIC SPACES 0AHW Contents 1. Introduction 1 2. Formal schemes à la EGA 2 3. Conventions and notation 8 4. Topological rings and modules 8 5. Affine formal algebraic spaces 14 6. Countably indexed affine formal algebraic spaces 19 7. Formal algebraic spaces 20 8. The reduction 22 9. Colimits of algebraic spaces along thickenings 24 10. Completion along a closed subset 26 11. Fibre products 29 12. Separation axioms for formal algebraic spaces 30 13. Quasi-compact formal algebraic spaces 32 14. Quasi-compact and quasi-separated formal algebraic spaces 32 15. Morphisms representable by algebraic spaces 34 16. Types of formal algebraic spaces 39 17. Morphisms and continuous ring maps 44 18. Taut ring maps and representability by algebraic spaces 49 19. Adic morphisms 50 20. Morphisms of finite type 51 21. Surjective morphisms 54 22. Monomorphisms 55 23. Closed immersions 55 24. Restricted power series 57 25. Algebras topologically of finite type 58 26. Separation axioms for morphisms 63 27. Proper morphisms 65 28. Formal algebraic spaces and fpqc coverings 66 29. Maps out of affine formal schemes 67 30. The small étale site of a formal algebraic space 69 31. The structure sheaf 72 32. Other chapters 74 References 75 1. Introduction 0AHX This is a chapter of the Stacks Project, version fac02ecd, compiled on Sep 14, 2021. 1 FORMAL ALGEBRAIC SPACES 2 Formal schemes were introduced in [DG67]. A more general version of formal schemes was introduced in [McQ02] and another in [Yas09]. Formal algebraic spaces were introduced in [Knu71]. Related material and much besides can be found in [Abb10] and [FK]. -
Lectures on Lubin-Tate Spaces Arizona Winter School March 2019 (Incomplete Draft)
Lectures on Lubin-Tate spaces Arizona Winter School March 2019 (Incomplete draft) M. J. Hopkins Department of Mathematics, Harvard University, Cambridge, MA 02138 Email address: [email protected] Contents Introduction5 Prerequisites5 Lecture 1. Lubin-Tate spaces7 1.1. Overview7 1.2. Height8 1.3. Deformations of formal groups9 1.4. Formal complex multiplication in local fields 10 1.5. Formal moduli for one-parameter formal Lie groups 13 1.6. Deformations a la Kodaira-Spencer 14 Lecture 2. Toward explicit formulas 17 2.1. Differentials and the log 17 2.2. p-typical coordinates 19 2.3. Hazewinkel's functional equation lemma 20 2.4. The Hazewinkel parameters 22 2.5. Dieudonn´emodules 23 2.6. Dieudonn´emodules and logarithms 25 2.7. Tapis de Cartier 26 2.8. Cleaning things up 29 Lecture 3. Crystals 31 3.1. A further formula 31 3.2. Periods 33 3.3. Crystals 36 3.4. The crystalline period map 46 Lecture 4. The crystalline approximation 49 Lecture 5. Projects 51 5.1. Project: finite subgroups 51 5.2. Project: Action of finite subgroups 52 5.3. Project: Line bundles in height 2 53 Bibliography 61 3 Introduction Lubin-Tate formal groups and their moduli play a surprising number of roles in mathematics, especially in number theory and algebraic topology. In number theory, the formal groups were originally used to construct the local class field theory isomorphism. Many years later a program emerged to use the moduli spaces to realize the local Langlands correspondence. In algebraic topology, thanks to Quillen, 1-dimensional commutative formal groups appear in the Adams-Novikov spectral sequence relating complex cobordism groups to stable homotopy groups, and through the work of Morava and many others, the Lubin-Tate groups lead to the \chromatic" picture of homotopy theory. -
Duality and Flat Base Change on Formal Schemes
Contemporary Mathematics Duality and Flat Base Change on Formal Schemes Leovigildo Alonso Tarr´ıo, Ana Jerem´ıas L´opez, and Joseph Lipman Abstract. We give several related versions of global Grothendieck Duality for unbounded complexes on noetherian formal schemes. The proofs, based on a non-trivial adaptation of Deligne's method for the special case of ordinary schemes, are reasonably self-contained, modulo the Special Adjoint Functor Theorem. An alternative approach, inspired by Neeman and based on recent results about \Brown Representability," is indicated as well. A section on applications and examples illustrates how our results synthesize a number of different duality-related topics (local duality, formal duality, residue theorems, dualizing complexes,. .). A flat-base-change theorem for pseudo-proper maps leads in particular to sheafified versions of duality for bounded-below complexes with quasi-coherent homology. Thanks to Greenlees-May duality, the results take a specially nice form for proper maps and bounded-below complexes with coherent homology. Contents 1. Preliminaries and main theorems. 4 2. Applications and examples. 10 3. Direct limits of coherent sheaves on formal schemes. 31 4. Global Grothendieck Duality. 43 5. Torsion sheaves. 47 6. Duality for torsion sheaves. 59 7. Flat base change. 71 8. Consequences of the flat base change isomorphism. 86 References 90 First two authors partially supported by Xunta de Galicia research project XUGA20701A96 and Spain's DGES grant PB97-0530. They also thank the Mathematics Department of Purdue University for its hospitality, help and support. Third author partially supported by the National Security Agency. c 1999 American Mathematical Society 3 4 LEOVIGILDO ALONSO, ANA JEREM´IAS, AND JOSEPH LIPMAN 1. -
Remarks on Nearby Cycles of Formal Schemes
Remarks on nearby cycles of formal schemes David Hansen⇤ May 9, 2018 Abstract We prove that the nearby cycles functor associated with the generic fiber of a p-adic for- mal scheme has many properties in common with derived pushforward along a proper map of schemes. In particular, we show that this functor has a right adjoint, satisfies the projection formula, and commutes with Verdier duality. Contents 1 Introduction 1 2 Proofs 4 2.1 Compact generation and the right adjoint ...................... 4 2.2 The projection formula and its consequences ..................... 5 2.3 Duality ......................................... 8 References 9 1 Introduction Let K be a nonarchimedean field, with ring of integers and residue field k,andletX be a O formal scheme over Spf which is locally of topologically finite type. By a classical construction of O Raynaud, there is a functorially associated rigid analytic generic fiber X = Xan, which we interpret as an adic space over Spa(K, ). There is a natural map of sites O λ : X X X ´e t ! ´e t corresponding to the functor sending an étale map Y X to the associated map on rigid generic ! fibers. Fix a Noetherian coefficient ring ⇤ killed by some positive integer, so λX induces a pushfor- ward functor λX on sheaves of ⇤-modules. We refer to the associated derived functor ⇤ RλX : D(X´e t , ⇤) D(X´e t , ⇤) ⇤ ! as the nearby cycles functor of X. This functor was introduced and studied (separately) by Berkovich and Huber [Ber94, Hub96], who established many of its important properties; in particular, when X ⇤Department of Mathematics, Columbia University, 2990 Broadway, New York NY 10027; [email protected] 1 is the formal completion of a finite type -scheme, they proved a comparison with classical nearby O cycles. -
Formal Group Laws
Formal group laws November 25, 2012 The additive formal group Ga plays a fundamental role in the theory. We will see in the first section that up to isomorphism this is the only formal group over a Q-algebra. This is far from true over other rings. The ring of endomorphisms of a formal group over a Q-algebra R is canonically iso- morphic to R. In general things are more complicated, even for the additive formal group. For example, as we will see in Section 3, the endomorphism ring of Ga over rings of characteristic p is quite interesting. It is related to the algebra of Steenrod reduced powers, familiar from algebraic topology. A wish to establish an analogue of Ga affording the full Steenrod algebra at an odd prime leads to the amusing project of working out the \super" analogue of formal groups, in Section 4. The final section is a misfit in this chapter: it is devoted to the relationship between the multiplicative group and gradings. 1 Formal group laws In studying formal groups one often picks a parameter and so obtains by transport of structure a \formal group law": Definition 1.1 A formal group law over R is a formal group over R whose underlying formal scheme is D. The group structure on D = Spf R[[t]] given by a formal group law is defined by an augmented (and hence automatically continuous) R-algebra homomorphism R[[t]] ! R[[t]]⊗^ R[[t]] = R[[u; v]] (where u = t ⊗ 1; v = 1 ⊗ t). This homomorphism is determined by the image of t, which is a formal power 1 series F (u; v) 2 R[[u; v]] such that F (0; v) = v; F (u; 0) = u; F (F (u; v); w) = F (u; F (v; w)); F (u; v) = F (v; u): It is easy to check inductively that there is a power series [−1]F (u) such that F ([−1]F (u); u) = 0 = F (u; [−1]F (u)), so we do not need to assume the existence of an inverse. -
Infinitesimal Local Study of Formal Schemes Leovigildo Alonso
Infinitesimal local study of formal schemes Leovigildo Alonso Tarr´ıo Ana Jerem´ıas L´opez Marta P´erez Rodr´ıguez Departamento de Alxebra,´ Facultade de Matematicas,´ Uni- versidade de Santiago de Compostela, E-15782 Santiago de Com- postela, SPAIN E-mail address: [email protected] URL: http://web.usc.es/~lalonso/ Current address: Department of Mathematics, Purdue University, West Lafayette IN 47907, USA Departamento de Alxebra,´ Facultade de Matematicas,´ Uni- versidade de Santiago de Compostela, E-15782 Santiago de Com- postela, SPAIN E-mail address: [email protected] Current address: Department of Mathematics, Purdue University, West Lafayette IN 47907, USA Departamento de Matematicas,´ Escola Superior de Enxene-˜ r´ıa Informatica,´ Campus de Ourense, Univ. de Vigo, E-32004 Ourense, Spain E-mail address: [email protected] arXiv:math/0504256v2 [math.AG] 10 May 2005 2000 Mathematics Subject Classification. Primary 14B10; Secondary 14A15, 14B20, 14B25, 14F10 Key words and phrases. formal scheme, infinitesimal lifting property, smooth morphism, unramified morphism, ´etale morphism, completion morphism, deformation. This work was partially supported by Spain’s MCyT and E.U.’s FEDER research project BFM2001-3241, supplemented by Xunta de Galicia grant PGDIT 01PX120701PR.. Abstract. We make a systematic study of the infinitesimal lifting con- ditions of a pseudo finite type map of noetherian formal schemes. We recover the usual general properties in this context, and, more impor- tantly, we uncover some new phenomena. We define a completion map of formal schemes as the one that arises canonically by performing the completion of a noetherian formal scheme along a subscheme, following the well-known pattern of ordinary schemes. -
Affine Grassmannians
BASICS ON AFFINE GRASSMANNIANS BY TIMO RICHARZ Contents 1. Ind-schemes 1 2. The affine Grassmannian for GLn 8 3. The affine Grassmannian for general groups 13 References 20 These are preliminary notes based on lectures given at the summer school `Buildings and affine Grassmannians' in Luminy, summer 2019. Comments are highly welcome! 1. Ind-schemes Ind-schemes provide a suitable language to handle the infinite-dimensional spaces such as the affine Grassmannians we encounter. Loosely speaking, an ind-scheme is a possibly infinite union 1 S i of schemes. The archetypical example is the infinite-dimensional affine space A = i≥0 A where Ai ⊂ Ai+1 via the first i coordinates. We explain our conventions and point out a few properties. Definition 1.1. A strict ind-scheme is a functor X : AffSchop ! Sets from the category of affine schemes which admits a presentation X ' colimi2I Xi as a filtered colimit of schemes where all transition maps Xi ! Xj, i ≤ j are closed immersions. The category of strict ind-schemes IndSch is the full subcategory of functors AffSchop ! Sets whose objects are strict ind-schemes. Here we regard schemes via the Yoneda embedding as a full subcategory of functors AffSchop ! Sets, and colimits of such functors are computed termwise. All ind-schemes in this survey are strict ind-schemes in the above sense, and we will usually drop the attribute `strict'. Also we identify AffSchop = Rings with the category of (commutative, unital) rings whenever convenient. Remark 1.2. Infact many ind-schemes of interest to us such as the affine Grassmannian admit a presentation X ' colimi2I Xi as in Definition 1.1 where the index set I is countable.