Class Field Theory
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Langlands, Weil, and Local Class Field Theory
LANGLANDS, WEIL, AND LOCAL CLASS FIELD THEORY DAVID SCHWEIN Abstract. The local Langlands conjecture predicts a relationship between Galois repre- sentations and representations of the topological general linear group. In this talk we explain a special case of the program, local class field theory, with a goal towards its nonabelian generalization. One of our main conclusions is that the Galois group should be replaced by a variant called the Weil group. As number theorists, we would like to understand the absolute Galois group ΓQ of Q. It is hard to say briefly why such an understanding would be interesting. But for one, if we understood the open, index-n subgroups of ΓQ then we would understand the number fields of degree n. There are much deeper reasons, related to the geometry of algebraic varieties, why we suspect ΓQ carries deep information, though that is more speculative. One way to study ΓQ is through its n-dimensional complex representations, that is, (con- jugacy classes of) homomorphisms ΓQ ! GLn(C): Starting with a 1967 letter to Weil, Langlands proposed that these representations are related to the representations of a completely different kind of group, the adelic group GLn(AQ). This circle of ideas is known as the Langlands program. The Langlands program over Q turned out to be very difficult. Even today, we cannot properly formulate a precise conjecture. So to get a start on the problem, mathematicians restricted attention to the completions of Q, namely Qp and R, and studied the problem there. As over Q, Langlands conjectured a relationship between n-dimensional Galois rep- resentations ΓQp ! GLn(C) and representations of the group GLn(Qp), as well as an analogous relationship over R. -
Determining Large Groups and Varieties from Small Subgroups and Subvarieties
Determining large groups and varieties from small subgroups and subvarieties This course will consist of two parts with the common theme of analyzing a geometric object via geometric sub-objects which are `small' in an appropriate sense. In the first half of the course, we will begin by describing the work on H. W. Lenstra, Jr., on finding generators for the unit groups of subrings of number fields. Lenstra discovered that from the point of view of computational complexity, it is advantageous to first consider the units of the ring of S-integers of L for some moderately large finite set of places S. This amounts to allowing denominators which only involve a prescribed finite set of primes. We will de- velop a generalization of Lenstra's method which applies to find generators of small height for the S-integral points of certain algebraic groups G defined over number fields. We will focus on G which are compact forms of GLd for d ≥ 1. In the second half of the course, we will turn to smooth projective surfaces defined by arithmetic lattices. These are Shimura varieties of complex dimension 2. We will try to generate subgroups of finite index in the fundamental groups of these surfaces by the fundamental groups of finite unions of totally geodesic projective curves on them, that is, via immersed Shimura curves. In both settings, the groups we will study are S-arithmetic lattices in a prod- uct of Lie groups over local fields. We will use the structure of our generating sets to consider several open problems about the geometry, group theory, and arithmetic of these lattices. -
Local Class Field Theory Via Lubin-Tate Theory and Applications
Local Class Field Theory via Lubin-Tate Theory and Applications Misja F.A. Steinmetz Supervisor: Prof. Fred Diamond June 2016 Abstract In this project we will introduce the study of local class field theory via Lubin-Tate theory following Yoshida [Yos08]. We explain how to construct the Artin map for Lubin-Tate extensions and we will show this map gives an isomorphism onto the Weil group of the maximal Lubin-Tate extension of our local field K: We will, furthermore, state (without proof) the other results needed to complete a proof of local class field theory in the classical sense. At the end of the project, we will look at a result from a recent preprint of Demb´el´e,Diamond and Roberts which gives an 1 explicit description of a filtration on H (GK ; Fp(χ)) for K a finite unramified extension of Qp and × χ : GK ! Fp a character. Using local class field theory, we will prove an analogue of this result for K a totally tamely ramified extension of Qp: 1 Contents 1 Introduction 3 2 Local Class Field Theory5 2.1 Formal Groups..........................................5 2.2 Lubin-Tate series.........................................6 2.3 Lubin-Tate Modules.......................................7 2.4 Lubin-Tate Extensions for OK ..................................8 2.5 Lubin-Tate Groups........................................9 2.6 Generalised Lubin-Tate Extensions............................... 10 2.7 The Artin Map.......................................... 11 2.8 Local Class Field Theory.................................... 13 1 3 Applications: Filtration on H (GK ; Fp(χ)) 14 3.1 Definition of the filtration.................................... 14 3.2 Computation of the jumps in the filtration.......................... -
Arithmetic Duality Theorems
Arithmetic Duality Theorems Second Edition J.S. Milne Copyright c 2004, 2006 J.S. Milne. The electronic version of this work is licensed under a Creative Commons Li- cense: http://creativecommons.org/licenses/by-nc-nd/2.5/ Briefly, you are free to copy the electronic version of the work for noncommercial purposes under certain conditions (see the link for a precise statement). Single paper copies for noncommercial personal use may be made without ex- plicit permission from the copyright holder. All other rights reserved. First edition published by Academic Press 1986. A paperback version of this work is available from booksellers worldwide and from the publisher: BookSurge, LLC, www.booksurge.com, 1-866-308-6235, [email protected] BibTeX information @book{milne2006, author={J.S. Milne}, title={Arithmetic Duality Theorems}, year={2006}, publisher={BookSurge, LLC}, edition={Second}, pages={viii+339}, isbn={1-4196-4274-X} } QA247 .M554 Contents Contents iii I Galois Cohomology 1 0 Preliminaries............................ 2 1 Duality relative to a class formation . ............. 17 2 Localfields............................. 26 3 Abelianvarietiesoverlocalfields.................. 40 4 Globalfields............................. 48 5 Global Euler-Poincar´echaracteristics................ 66 6 Abelianvarietiesoverglobalfields................. 72 7 An application to the conjecture of Birch and Swinnerton-Dyer . 93 8 Abelianclassfieldtheory......................101 9 Otherapplications..........................116 AppendixA:Classfieldtheoryforfunctionfields............126 -
Number Theory
Number Theory Alexander Paulin October 25, 2010 Lecture 1 What is Number Theory Number Theory is one of the oldest and deepest Mathematical disciplines. In the broadest possible sense Number Theory is the study of the arithmetic properties of Z, the integers. Z is the canonical ring. It structure as a group under addition is very simple: it is the infinite cyclic group. The mystery of Z is its structure as a monoid under multiplication and the way these two structure coalesce. As a monoid we can reduce the study of Z to that of understanding prime numbers via the following 2000 year old theorem. Theorem. Every positive integer can be written as a product of prime numbers. Moreover this product is unique up to ordering. This is 2000 year old theorem is the Fundamental Theorem of Arithmetic. In modern language this is the statement that Z is a unique factorization domain (UFD). Another deep fact, due to Euclid, is that there are infinitely many primes. As a monoid therefore Z is fairly easy to understand - the free commutative monoid with countably infinitely many generators cross the cyclic group of order 2. The point is that in isolation addition and multiplication are easy, but together when have vast hidden depth. At this point we are faced with two potential avenues of study: analytic versus algebraic. By analytic I questions like trying to understand the distribution of the primes throughout Z. By algebraic I mean understanding the structure of Z as a monoid and as an abelian group and how they interact. -
Algebraic Number Theory II
Algebraic number theory II Uwe Jannsen Contents 1 Infinite Galois theory2 2 Projective and inductive limits9 3 Cohomology of groups and pro-finite groups 15 4 Basics about modules and homological Algebra 21 5 Applications to group cohomology 31 6 Hilbert 90 and Kummer theory 41 7 Properties of group cohomology 48 8 Tate cohomology for finite groups 53 9 Cohomology of cyclic groups 56 10 The cup product 63 11 The corestriction 70 12 Local class field theory I 75 13 Three Theorems of Tate 80 14 Abstract class field theory 83 15 Local class field theory II 91 16 Local class field theory III 94 17 Global class field theory I 97 0 18 Global class field theory II 101 19 Global class field theory III 107 20 Global class field theory IV 112 1 Infinite Galois theory An algebraic field extension L/K is called Galois, if it is normal and separable. For this, L/K does not need to have finite degree. For example, for a finite field Fp with p elements (p a prime number), the algebraic closure Fp is Galois over Fp, and has infinite degree. We define in this general situation Definition 1.1 Let L/K be a Galois extension. Then the Galois group of L over K is defined as Gal(L/K) := AutK (L) = {σ : L → L | σ field automorphisms, σ(x) = x for all x ∈ K}. But the main theorem of Galois theory (correspondence between all subgroups of Gal(L/K) and all intermediate fields of L/K) only holds for finite extensions! To obtain the correct answer, one needs a topology on Gal(L/K): Definition 1.2 Let L/K be a Galois extension. -
Weil Groups and $ F $-Isocrystals
Weil groups and F -Isocrystals Richard Crew The University of Florida December 19, 2017 Introduction The starting point for local class field theory in most modern treatments is the classification of central simple algebras over a local field K. On the other hand an old theorem of Dieudonn´e[6] may interpreted as saying that when K is absolutely unramified and of characteristic zero any such algebra is the endomorphism algebra of an isopentic F -isocrystal on the completion Knr of a maximal unramified extension of K. In fact the restrictions on K can be removed by a suitably generalizing the notion of F -isocrystal, so one is led to ask if there approach to local class field theory based on the structure theory of F -isocrystals. One aim of this article is to carry out this project, and we will find that it gives a particularly quick approach to most of the main results of local class field theory, the exception being the existence theorem. We will not need many of the more technical results of group cohomology such as the Tate-Nakayama theorem or the “ugly lemma” of [4, Ch. 6 §1.5], and the formal properties of the norm residue symbol follow from the construction rather than the usual cohomological formalism. In this approach one sees a close relation between two topics not normally associated with each other. The first is a celebrated formula of Dwork for the norm residue symbol in the case of a totally ramified abelian extension. A generalization of Dwork’s formula valid for any finite Galois extension can be unearthed from the exercises in chapter XIII of [10]. -
On the Non-Abelian Global Class Field Theory
See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/268165110 On the non-abelian global class field theory Article · September 2013 DOI: 10.1007/s40316-013-0004-9 CITATION READS 1 19 1 author: Kazım Ilhan Ikeda Bogazici University 15 PUBLICATIONS 28 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: The Langlands Reciprocity and Functoriality Principles View project All content following this page was uploaded by Kazım Ilhan Ikeda on 10 January 2018. The user has requested enhancement of the downloaded file. On the non-abelian global class field theory Kazımˆ Ilhan˙ Ikeda˙ –Dedicated to Robert Langlands for his 75th birthday – Abstract. Let K be a global field. The aim of this speculative paper is to discuss the possibility of constructing the non-abelian version of global class field theory of K by “glueing” the non-abelian local class field theories of Kν in the sense of Koch, for each ν ∈ hK, following Chevalley’s philosophy of ideles,` and further discuss the relationship of this theory with the global reciprocity principle of Langlands. Mathematics Subject Classification (2010). Primary 11R39; Secondary 11S37, 11F70. Keywords. global fields, idele` groups, global class field theory, restricted free products, non-abelian idele` groups, non-abelian global class field theory, ℓ-adic representations, automorphic representations, L-functions, Langlands reciprocity principle, global Langlands groups. 1. Non-abelian local class field theory in the sense of Koch For details on non-abelian local class field theory (in the sense of Koch), we refer the reader to the papers [21, 23, 24, 50] as well as Laubie’s work [37]. -
Questions and Remarks to the Langlands Program
Questions and remarks to the Langlands program1 A. N. Parshin (Uspekhi Matem. Nauk, 67(2012), n 3, 115-146; Russian Mathematical Surveys, 67(2012), n 3, 509-539) Introduction ....................................... ......................1 Basic fields from the viewpoint of the scheme theory. .............7 Two-dimensional generalization of the Langlands correspondence . 10 Functorial properties of the Langlands correspondence. ................12 Relation with the geometric Drinfeld-Langlands correspondence . 16 Direct image conjecture . ...................20 A link with the Hasse-Weil conjecture . ................27 Appendix: zero-dimensional generalization of the Langlands correspondence . .................30 References......................................... .....................33 Introduction The goal of the Langlands program is a correspondence between representations of the Galois groups (and their generalizations or versions) and representations of reductive algebraic groups. The starting point for the construction is a field. Six types of the fields are considered: three types of local fields and three types of global fields [L3, F2]. The former ones are the following: 1) finite extensions of the field Qp of p-adic numbers, the field R of real numbers and the field C of complex numbers, arXiv:1307.1878v1 [math.NT] 7 Jul 2013 2) the fields Fq((t)) of Laurent power series, where Fq is the finite field of q elements, 3) the field of Laurent power series C((t)). The global fields are: 4) fields of algebraic numbers (= finite extensions of the field Q of rational numbers), 1I am grateful to R. P. Langlands for very useful conversations during his visit to the Steklov Mathematical institute of the Russian Academy of Sciences (Moscow, October 2011), to Michael Harris and Ulrich Stuhler, who answered my sometimes too naive questions, and to Ilhan˙ Ikeda˙ who has read a first version of the text and has made several remarks. -
22 Ring Class Fields and the CM Method
18.783 Elliptic Curves Spring 2015 Lecture #22 04/30/2015 22 Ring class fields and the CM method p Let O be an imaginary quadratic order of discriminant D, let K = Q( D), and let L be the splitting field of the Hilbert class polynomial HD(X) over K. In the previous lecture we showed that there is an injective group homomorphism Ψ: Gal(L=K) ,! cl(O) that commutes with the group actions of Gal(L=K) and cl(O) on the set EllO(C) = EllO(L) of roots of HD(X) (the j-invariants of elliptic curves with CM by O). To complete the proof of the the First Main Theorem of Complex Multiplication, which asserts that Ψ is an isomorphism, we need to show that Ψ is surjective; this is equivalent to showing the HD(X) is irreducible over K. At the end of the last lecture we introduced the Artin map p 7! σp, which sends each unramified prime p of K to the unique automorphism σp 2 Gal(L=K) for which Np σp(x) ≡ x mod q; (1) for all x 2 OL and primes q of L dividing pOL (recall that σp is independent of q because Gal(L=K) ,! cl(O) is abelian). Equivalently, σp is the unique element of Gal(L=K) that Np fixes q and induces the Frobenius automorphism x 7! x of Fq := OL=q, which is a generator for Gal(Fq=Fp), where Fp := OK =p. Note that if E=C has CM by O then j(E) 2 L, and this implies that E can be defined 2 3 by a Weierstrass equation y = x + Ax + B with A; B 2 OL. -
22 Ring Class Fields and the CM Method
18.783 Elliptic Curves Spring 2017 Lecture #22 05/03/2017 22 Ring class fields and the CM method Let O be an imaginary quadratic order of discriminant D, and let EllO(C) := fj(E) 2 C : End(E) = Cg. In the previous lecture we proved that the Hilbert class polynomial Y HD(X) := HO(X) := X − j(E) j(E)2EllO(C) has integerp coefficients. We then defined L to be the splitting field of HD(X) over the field K = Q( D), and showed that there is an injective group homomorphism Ψ: Gal(L=K) ,! cl(O) that commutes with the group actions of Gal(L=K) and cl(O) on the set EllO(C) = EllO(L) of roots of HD(X). To complete the proof of the the First Main Theorem of Complex Multiplication, which asserts that Ψ is an isomorphism, we need to show that Ψ is surjective, equivalently, that HD(X) is irreducible over K. At the end of the last lecture we introduced the Artin map p 7! σp, which sends each unramified prime p of K (prime ideal of OK ) to the corresponding Frobenius element σp, which is the unique element of Gal(L=K) for which Np σp(x) ≡ x mod q; (1) for all x 2 OL and primes qjp (prime ideals of OL that divide the ideal pOL); the existence of a single σp 2 Gal(L=K) satisfying (1) for all qjp follows from the fact that Gal(L=K) ,! cl(O) is abelian. The Frobenius element σp can also be characterized as follows: for each prime qjp the finite field Fq := OL=q is an extension of the finite field Fp := OK =p and the automorphism σ¯p 2 Gal(Fq=Fp) defined by σ¯p(¯x) = σ(x) (where x 7! x¯ is the reduction Np map OL !OL=q), is the Frobenius automorphism x 7! x generating Gal(Fq=Fp). -
The Twelfth Problem of Hilbert Reminds Us, Although the Reminder Should
Some Contemporary Problems with Origins in the Jugendtraum Robert P. Langlands The twelfth problem of Hilbert reminds us, although the reminder should be unnecessary, of the blood relationship of three subjects which have since undergone often separate devel• opments. The first of these, the theory of class fields or of abelian extensions of number fields, attained what was pretty much its final form early in this century. The second, the algebraic theory of elliptic curves and, more generally, of abelian varieties, has been for fifty years a topic of research whose vigor and quality shows as yet no sign of abatement. The third, the theory of automorphic functions, has been slower to mature and is still inextricably entangled with the study of abelian varieties, especially of their moduli. Of course at the time of Hilbert these subjects had only begun to set themselves off from the general mathematical landscape as separate theories and at the time of Kronecker existed only as part of the theories of elliptic modular functions and of cyclotomicfields. It is in a letter from Kronecker to Dedekind of 1880,1 in which he explains his work on the relation between abelian extensions of imaginary quadratic fields and elliptic curves with complex multiplication, that the word Jugendtraum appears. Because these subjects were so interwoven it seems to have been impossible to disentangle the different kinds of mathematics which were involved in the Jugendtraum, especially to separate the algebraic aspects from the analytic or number theoretic. Hilbert in particular may have been led to mistake an accident, or perhaps necessity, of historical development for an “innigste gegenseitige Ber¨uhrung.” We may be able to judge this better if we attempt to view the mathematical content of the Jugendtraum with the eyes of a sophisticated contemporary mathematician.