EXAMPLES for the MOD P MOTIVIC COHOMOLOGY of CLASSIFYING SPACES
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Slices of Hermitian K-Theory and Milnor's Conjecture on Quadratic Forms
Slices of hermitian K-theory and Milnor’s conjecture on quadratic forms Oliver R¨ondigs Paul Arne Østvær October 15, 2018 Abstract We advance the understanding of K-theory of quadratic forms by computing the slices of the motivic spectra representing hermitian K-groups and Witt-groups. By an explicit computation of the slice spectral sequence for higher Witt-theory, we prove Milnor’s conjecture relating Galois cohomology to quadratic forms via the filtration of the Witt ring by its fundamental ideal. In a related computation we express hermitian K-groups in terms of motivic cohomology. Contents 1 Introduction 2 2 Slices and the slice spectral sequence 4 3 Algebraic and hermitian K-theory 8 4 Slices 11 4.1 Algebraic K-theory..................................... 11 4.2 Homotopy orbit K-theory ................................. 11 4.3 Hermitian K-theory .................................... 14 4.4 Themysterioussummandistrivial . ......... 17 4.5 HigherWitt-theory............................... ...... 21 5 Differentials 21 5.1 Mod-2 algebraic K-theory................................. 21 arXiv:1311.5833v2 [math.KT] 30 May 2017 5.2 Hermitian K-theory,I ................................... 23 5.3 HigherWitt-theory............................... ...... 25 5.4 Hermitian K-theory,II................................... 27 6 Milnor’s conjecture on quadratic forms 28 7 Hermitian K-groups 36 A Maps between motivic Eilenberg-MacLane spectra 40 1 1 Introduction Suppose that F is a field of characteristic char(F ) = 2. In [33] the Milnor K-theory of F is defined 6 in terms of generators and relations by KM (F )= T ∗F ×/(a (1 a)); a = 0, 1. ∗ ⊗ − 6 Here T ∗F × is the tensor algebra of the multiplicative group of units F ×. -
What Is the Motivation Behind the Theory of Motives?
What is the motivation behind the Theory of Motives? Barry Mazur How much of the algebraic topology of a connected simplicial complex X is captured by its one-dimensional cohomology? Specifically, how much do you know about X when you know H1(X, Z) alone? For a (nearly tautological) answer put GX := the compact, connected abelian Lie group (i.e., product of circles) which is the Pontrjagin dual of the free abelian group H1(X, Z). Now H1(GX, Z) is canonically isomorphic to H1(X, Z) = Hom(GX, R/Z) and there is a canonical homotopy class of mappings X −→ GX which induces the identity mapping on H1. The answer: we know whatever information can be read off from GX; and are ignorant of anything that gets lost in the projection X → GX. The theory of Eilenberg-Maclane spaces offers us a somewhat analogous analysis of what we know and don’t know about X, when we equip ourselves with n-dimensional cohomology, for any specific n, with specific coefficients. If we repeat our rhetorical question in the context of algebraic geometry, where the structure is somewhat richer, can we hope for a similar discussion? In algebraic topology, the standard cohomology functor is uniquely characterized by the basic Eilenberg-Steenrod axioms in terms of a simple normalization (the value of the functor on a single point). In contrast, in algebraic geometry we have a more intricate set- up to deal with: for one thing, we don’t even have a cohomology theory with coefficients in Z for varieties over a field k unless we provide a homomorphism k → C, so that we can form the topological space of complex points on our variety, and compute the cohomology groups of that topological space. -
CATERADS and MOTIVIC COHOMOLOGY Contents Introduction 1 1. Weighted Multiplicative Presheaves 3 2. the Standard Motivic Cochain
CATERADS AND MOTIVIC COHOMOLOGY B. GUILLOU AND J.P. MAY Abstract. We define “weighted multiplicative presheaves” and observe that there are several weighted multiplicative presheaves that give rise to motivic cohomology. By neglect of structure, weighted multiplicative presheaves give symmetric monoids of presheaves. We conjecture that a suitable stabilization of one of the symmetric monoids of motivic cochain presheaves has an action of a caterad of presheaves of acyclic cochain complexes, and we give some fragmentary evidence. This is a snapshot of work in progress. Contents Introduction 1 1. Weighted multiplicative presheaves 3 2. The standard motivic cochain complex 5 3. A variant of the standard motivic complex 7 4. The Suslin-Friedlander motivic cochain complex 8 5. Towards caterad actions 10 6. The stable linear inclusions caterad 11 7. Steenrod operations? 13 References 14 Introduction The first four sections set the stage for discussion of a conjecture about the multiplicative structure of motivic cochain complexes. In §1, we define the notion of a “weighted multiplicative presheaf”, abbreviated WMP. This notion codifies formal structures that appear naturally in motivic cohomology and presumably elsewhere in algebraic geometry. We observe that WMP’s determine symmetric monoids of presheaves, as defined in [11, 1.3], by neglect of structure. In §§2–4, we give an outline summary of some of the constructions of motivic co- homology developed in [13, 17] and state the basic comparison theorems that relate them to each other and to Bloch’s higher Chow complexes. There is a labyrinth of definitions and comparisons in this area, and we shall just give a brief summary overview with emphasis on product structures. -
The Spectral Sequence Relating Algebraic K-Theory to Motivic Cohomology
Ann. Scient. Éc. Norm. Sup., 4e série, t. 35, 2002, p. 773 à 875. THE SPECTRAL SEQUENCE RELATING ALGEBRAIC K-THEORY TO MOTIVIC COHOMOLOGY BY ERIC M. FRIEDLANDER 1 AND ANDREI SUSLIN 2 ABSTRACT. – Beginning with the Bloch–Lichtenbaum exact couple relating the motivic cohomology of a field F to the algebraic K-theory of F , the authors construct a spectral sequence for any smooth scheme X over F whose E2 term is the motivic cohomology of X and whose abutment is the Quillen K-theory of X. A multiplicative structure is exhibited on this spectral sequence. The spectral sequence is that associated to a tower of spectra determined by consideration of the filtration of coherent sheaves on X by codimension of support. 2002 Éditions scientifiques et médicales Elsevier SAS RÉSUMÉ. – Partant du couple exact de Bloch–Lichtenbaum, reliant la cohomologie motivique d’un corps F àsaK-théorie algébrique, on construit, pour tout schéma lisse X sur F , une suite spectrale dont le terme E2 est la cohomologie motivique de X et dont l’aboutissement est la K-théorie de Quillen de X. Cette suite spectrale, qui possède une structure multiplicative, est associée à une tour de spectres déterminée par la considération de la filtration des faisceaux cohérents sur X par la codimension du support. 2002 Éditions scientifiques et médicales Elsevier SAS The purpose of this paper is to establish in Theorem 13.6 a spectral sequence from the motivic cohomology of a smooth variety X over a field F to the algebraic K-theory of X: p,q p−q Z − −q − − ⇒ (13.6.1) E2 = H X, ( q) = CH (X, p q) K−p−q(X). -
Detecting Motivic Equivalences with Motivic Homology
Detecting motivic equivalences with motivic homology David Hemminger December 7, 2020 Let k be a field, let R be a commutative ring, and assume the exponential characteristic of k is invertible in R. Let DM(k; R) denote Voevodsky’s triangulated category of motives over k with coefficients in R. In a failed analogy with topology, motivic homology groups do not detect iso- morphisms in DM(k; R) (see Section 1). However, it is often possible to work in a context partially agnostic to the base field k. In this note, we prove a detection result for those circumstances. Theorem 1. Let ϕ: M → N be a morphism in DM(k; R). Suppose that either a) For every separable finitely generated field extension F/k, the induced map on motivic homology H∗(MF ,R(∗)) → H∗(NF ,R(∗)) is an isomorphism, or b) Both M and N are compact, and for every separable finitely generated field exten- ∗ ∗ sion F/k, the induced map on motivic cohomology H (NF ,R(∗)) → H (MF ,R(∗)) is an isomorphism. Then ϕ is an isomorphism. Note that if X is a separated scheme of finite type over k, then the motive M(X) of X in DM(k; R) is compact, by [6, Theorem 11.1.13]. The proof is easy: following a suggestion of Bachmann, we reduce to the fact (Proposition 5) that a morphism in SH(k) which induces an isomorphism on homo- topy sheaves evaluated on all separable finitely generated field extensions must be an isomorphism. This follows immediately from Morel’s work on unramified presheaves (see [9]). -
Motivic Cohomology, K-Theory and Topological Cyclic Homology
Motivic cohomology, K-theory and topological cyclic homology Thomas Geisser? University of Southern California, [email protected] 1 Introduction We give a survey on motivic cohomology, higher algebraic K-theory, and topo- logical cyclic homology. We concentrate on results which are relevant for ap- plications in arithmetic algebraic geometry (in particular, we do not discuss non-commutative rings), and our main focus lies on sheaf theoretic results for smooth schemes, which then lead to global results using local-to-global methods. In the first half of the paper we discuss properties of motivic cohomology for smooth varieties over a field or Dedekind ring. The construction of mo- tivic cohomology by Suslin and Voevodsky has several technical advantages over Bloch's construction, in particular it gives the correct theory for singu- lar schemes. But because it is only well understood for varieties over fields, and does not give well-behaved ´etalemotivic cohomology groups, we discuss Bloch's higher Chow groups. We give a list of basic properties together with the identification of the motivic cohomology sheaves with finite coefficients. In the second half of the paper, we discuss algebraic K-theory, ´etale K- theory and topological cyclic homology. We sketch the definition, and give a list of basic properties of algebraic K-theory, sketch Thomason's hyper- cohomology construction of ´etale K-theory, and the construction of topolog- ical cyclic homology. We then give a short overview of the sheaf theoretic properties, and relationships between the three theories (in many situations, ´etale K-theory with p-adic coefficients and topological cyclic homology agree). -
A1-Algebraic Topology
A1-algebraic topology Fabien Morel Abstract. We present some recent results in A1-algebraic topology, which means both in A1-homotopy theory of schemes and its relationship with algebraic geometry. This refers to the classical relationship between homotopy theory and (differential) topology. We explain several examples of “motivic” versions of classical results: the theory of the Brouwer degree, the classification of A1-coverings through the A1-fundamental group, the Hurewicz Theorem and the A1-homotopy of algebraic spheres, and the A1-homotopy classification of vector bundles. We also give some applications and perspectives. Mathematics Subject Classification (2000). 14F05, 19E15, 55P. Keywords. A1-homotopy theory, Milnor K-theory, Witt groups. 1. The Brouwer degree Let n ≥ 1 be an integer and let X be a pointed topological space. We shall denote by πn(X) the n-th homotopy group of X. A basic fact in homotopy theory is: Theorem 1.1. Let n ≥ 1, d ≥ 1 be integers and denote by Sn the n-dimensional sphere. n 1) If d<nthen πd (S ) = 0; n 2) If d = n then πn(S ) = Z. A classical proof uses the Hurewicz Theorem and the computation of the integral singular homology of the sphere. Half of this paper is devoted to explain the analogue of these results in A1-homotopy theory [54], [38]. For our purpose we also recall a more geometric proof of 2) inspired by the definition of Brouwer’s degree. Any continuous map Sn → Sn is homotopic to a C∞-differentiable map f : Sn → Sn. By Sard’s theorem, f has at least one regular value x ∈ Sn, so that f −1(x) is a finite set of points in Sn and for each y ∈ f −1(x), the n n differential dfy : Ty(S ) → Tx(S ) of f at y is an isomorphism. -
Motivic Homotopy Theory Dexter Chua
Motivic Homotopy Theory Dexter Chua 1 The Nisnevich topology 2 2 A1-localization 4 3 Homotopy sheaves 5 4 Thom spaces 6 5 Stable motivic homotopy theory 7 6 Effective and very effective motivic spectra 9 Throughout the talk, S is a quasi-compact and quasi-separated scheme. Definition 0.1. Let SmS be the category of finitely presented smooth schemes over S. Since we impose the finitely presented condition, this is an essentially small (1-)category. The starting point of motivic homotopy theory is the 1-category P(SmS) of presheaves on SmS. This is a symmetric monoidal category under the Cartesian product. Eventually, we will need the pointed version P(SmS)∗, which can be defined either as the category of pointed objects in P(SmS), or the category of presheaves with values in pointed spaces. This is symmetric monoidal 1-category under the (pointwise) smash product. In the first two chapters, we will construct the unstable motivic category, which fits in the bottom-right corner of the following diagram: LNis P(SmS) LNisP(SmS) ≡ SpcS L 1 Lg1 A A 1 LgNis 1 1 A LA P(SmS) LA ^NisP(SmS) ≡ SpcS ≡ H(S) The first chapter will discuss the horizontal arrows (i.e. Nisnevich localization), and the second will discuss the vertical ones (i.e. A1-localization). 1 Afterwards, we will start \doing homotopy theory". In chapter 3, we will discuss the motivic version of homotopy groups, and in chapter 4, we will discuss Thom spaces. In chapter 5, we will introduce the stable motivic category, and finally, in chapter 6, we will introduce effective and very effective spectra. -
Introduction to Motives
Introduction to motives Sujatha Ramdorai and Jorge Plazas With an appendix by Matilde Marcolli Abstract. This article is based on the lectures of the same tittle given by the first author during the instructional workshop of the program \number theory and physics" at ESI Vienna during March 2009. An account of the topics treated during the lectures can be found in [24] where the categorical aspects of the theory are stressed. Although naturally overlapping, these two independent articles serve as complements to each other. In the present article we focus on the construction of the category of pure motives starting from the category of smooth projective varieties. The necessary preliminary material is discussed. Early accounts of the theory were given in Manin [21] and Kleiman [19], the material presented here reflects to some extent their treatment of the main aspects of the theory. We also survey the theory of endomotives developed in [5], this provides a link between the theory of motives and tools from quantum statistical mechanics which play an important role in results connecting number theory and noncommutative geometry. An extended appendix (by Matilde Marcolli) further elaborates these ideas and reviews the role of motives in noncommutative geometry. Introduction Various cohomology theories play a central role in algebraic geometry, these co- homology theories share common properties and can in some cases be related by specific comparison morphisms. A cohomology theory with coefficients in a ring R is given by a contra-variant functor H from the category of algebraic varieties over a field k to the category of graded R-algebras (or more generally to a R-linear tensor category). -
Stable Motivic Invariants Are Eventually Étale Local
STABLE MOTIVIC INVARIANTS ARE EVENTUALLY ETALE´ LOCAL TOM BACHMANN, ELDEN ELMANTO, AND PAUL ARNE ØSTVÆR Abstract. We prove a Thomason-style descent theorem for the motivic sphere spectrum, and deduce an ´etale descent result applicable to all motivic spectra. To this end, we prove a new convergence result for the slice spectral sequence, following work by Levine and Voevodsky. This generalizes and extends previous ´etale descent results for special examples of motivic cohomology theories. Combined with ´etale rigidity results, we obtain a complete structural description of the ´etale motivic stable category. Contents 1. Introduction 2 1.1. What is done in this paper? 2 1.2. Bott elements and multiplicativity of the Moore spectrum 4 1.3. Some applications 4 1.4. Overview of ´etale motivic cohomology theories 4 1.5. Terminology and notation 5 1.6. Acknowledgements 6 2. Preliminaries 6 2.1. Endomorphisms of the motivic sphere 6 2.2. ρ-completion 7 2.3. Virtual ´etale cohomological dimension 8 3. Bott-inverted spheres 8 3.1. τ-self maps 8 3.2. Cohomological Bott elements 9 3.3. Spherical Bott elements 10 4. Construction of spherical Bott elements 10 4.1. Construction via roots of unity 10 4.2. Lifting to higher ℓ-powers 10 4.3. Construction by descent 11 4.4. Construction over special fields 12 4.5. Summary of τ-self maps 13 5. Slice convergence 13 6. Spheres over fields 18 7. Main result 19 Appendix A. Multiplicative structures on Moore objects 23 A.1. Definitions and setup 23 A.2. Review of Oka’s results 23 arXiv:2003.04006v2 [math.KT] 20 Mar 2020 A.3. -
MILNOR K-THEORY and MOTIVIC COHOMOLOGY 1. Introduction A
MILNOR K-THEORY AND MOTIVIC COHOMOLOGY MORITZ KERZ Abstract. These are the notes of a talk given at the Oberwolfach Workshop K-Theory 2006. We sketch a proof of Beilinson’s conjecture relating Milnor K-theory and motivic cohomology. For detailed proofs see [4]. 1. Introduction A — semi-local commutative ring with infinite residue fields k — field Z(n) — Voevodsky’s motivic complex [8] Definition 1.1. M M × ⊗n × K∗ (A) = (A ) /(a ⊗ (1 − a)) a, 1 − a ∈ A n Beilinson conjectured [1]: Theorem 1.2. A/k essentially smooth, |k| = ∞. Then: M n η : Kn (A) −→ Hzar(A, Z(n)) is an isomorphism for n > 0. The formerly known cases are: Remark 1.3. — A=k a field (Nesterenko-Suslin [6], Totaro [10]) — surjectivity of η (Gabber [3], Elbaz-Vincent/M¨uller-Stach [2], Kerz/M¨uller-Stach [5]) — η ⊗ Q is isomorphic (Suslin) — injectivity for A a DVR, n = 3 (Suslin-Yarosh [9]) 2. General idea of proof X = Spec A We have a morphism of Gersten complexes which we know to be exact except possibly M at Kn (A): Date: 7/20/06. The author is supported by Studienstiftung des deutschen Volkes. 1 2 MORITZ KERZ (1) M M M 0 / Kn (A) / ⊕x∈X(0) Kn (x) / ⊕x∈X(1) Kn−1(x) n n n−1 0 / Hzar(A), Z(n)) / ⊕x∈X(0) Hzar(x, Z(n)) / ⊕x∈X(1) Hzar (x, Z(n − 1)) So it suffices to prove: Theorem 2.1 (Main Result). A/k regular, connected, infinite residue fields, F = Q(A). -
Motivic Homotopy Theory
Voevodsky’s Nordfjordeid Lectures: Motivic Homotopy Theory Vladimir Voevodsky1, Oliver R¨ondigs2, and Paul Arne Østvær3 1 School of Mathematics, Institute for Advanced Study, Princeton, USA [email protected] 2 Fakult¨at f¨ur Mathematik, Universit¨atBielefeld, Bielefeld, Germany [email protected] 3 Department of Mathematics, University of Oslo, Oslo, Norway [email protected] 148 V. Voevodsky et al. 1 Introduction Motivic homotopy theory is a new and in vogue blend of algebra and topology. Its primary object is to study algebraic varieties from a homotopy theoretic viewpoint. Many of the basic ideas and techniques in this subject originate in algebraic topology. This text is a report from Voevodsky’s summer school lectures on motivic homotopy in Nordfjordeid. Its first part consists of a leisurely introduction to motivic stable homotopy theory, cohomology theories for algebraic varieties, and some examples of current research problems. As background material, we recommend the lectures of Dundas [Dun] and Levine [Lev] in this volume. An introductory reference to motivic homotopy theory is Voevodsky’s ICM address [Voe98]. The appendix includes more in depth background material required in the main body of the text. Our discussion of model structures for motivic spectra follows Jardine’s paper [Jar00]. In the first part, we introduce the motivic stable homotopy category. The examples of motivic cohomology, algebraic K-theory, and algebraic cobordism illustrate the general theory of motivic spectra. In March 2000, Voevodsky [Voe02b] posted a list of open problems concerning motivic homotopy theory. There has been so much work done in the interim that our update of the status of these conjectures may be useful to practitioners of motivic homotopy theory.