Arithmetic of Linear Algebraic Groups Over Two-Dimensional Geometric fields
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Minimal Weakly Isotropic Forms
Minimal weakly isotropic forms Karim Johannes Becher Abstract In this article weakly isotropic quadratic forms over a (formally) real field are studied. Conditions on the field are given which imply that every weakly isotropic form over that field has a weakly isotropic sub- form of small dimension. Fields over which every quadratic form can be decomposed into an orthogonal sum of a strongly anisotropic form and a torsion form are characterized in different ways. Keywords: quadratic form, weakly isotropic, strongly anisotropic, real field, Pythagoras number, field invariants, SAP-field, local-global principle Classification (MSC 2000): 11E04, 11E81, 12D15 1 Introduction The concept of weak isotropy was introduced by Prestel in [11] and studied in the 1970ies, especially by Prestel and Br¨ocker. The main motivation for this concept is the failure of any kind of local-global principle for isotropy of quadratic forms in a general situation. It turned out that, over any (formally) real field, weakly isotropic forms can be characterized, on the one hand in terms of the semi-orderings (cf. [11]), on the other hand in terms of the real places of the field (cf. [11], [2]). Applications of weakly isotropic forms occur in real algebra and geometry (cf. [13]). The present article investigates weakly isotropic forms in view of the question what kind of subforms and orthogonal decompositions they admit. This leads to consider in particular those weakly isotropic forms which are minimal in the sense that they do not contain any proper subform which is weakly isotropic. The question of how large the dimension of such a form may be gives rise to a new field invariant. -
Differential Forms, Linked Fields and the $ U $-Invariant
Differential Forms, Linked Fields and the u-Invariant Adam Chapman Department of Computer Science, Tel-Hai Academic College, Upper Galilee, 12208 Israel Andrew Dolphin Department of Mathematics, Ghent University, Ghent, Belgium Abstract We associate an Albert form to any pair of cyclic algebras of prime degree p over a field F with char(F) = p which coincides with the classical Albert form when p = 2. We prove that if every Albert form is isotropic then H4(F) = 0. As a result, we obtain that if F is a linked field with char(F) = 2 then its u-invariant is either 0, 2, 4or 8. Keywords: Differential Forms, Quadratic Forms, Linked Fields, u-Invariant, Fields of Finite Characteristic. 2010 MSC: 11E81 (primary); 11E04, 16K20 (secondary) 1. Introduction Given a field F, a quaternion algebra over F is a central simple F-algebra of degree 2. The maximal subfields of quaternion division algebras over F are quadratic field extensions of F. When char(F) , 2, all quadratic field extensions are separable. When char(F) = 2, there are two types of quadratic field extensions: the separable type which is of the form F[x : x2 + x = α] for some α F λ2 + λ : λ F , and the ∈ \ { 2 ∈ } inseparable type which is of the form F[ √α] for some α F× (F×) . In this case, any quaternion division algebra contains both types of field ext∈ ensions,\ which can be seen by its symbol presentation 2 2 1 [α, β)2,F = F x, y : x + x = α, y = β, yxy− = x + 1 . arXiv:1701.01367v2 [math.RA] 22 May 2017 h i If char(F) = p for some prime p > 0, we let ℘(F) denote the additive subgroup λp λ : λ F . -
Serre's Conjecture II: a Survey
Serre’s conjecture II: a survey Philippe Gille Dedicated to Parimala Abstract We provide a survey of Serre’s conjecture II (1962) on the vanishing of Galois cohomology for simply connected semisimple groups defined over a field of cohomological dimension 2. ≤ Math subject classification: 20G05. 1 Introduction Serre’s original conjecture II (1962) states that the Galois cohomology set H1(k,G) vanishes for a semisimple simply connected algebraic group G defined over a perfect field of cohomological dimension 2 [53, 4.1] [54, II.3.1]. This means that G- ≤ § torsors (or principal homogeneous spaces) over Spec(k) are trivial. For example, if A is a central simple algebra defined over a field k and c k , ∈ × the subvariety X := nrd(y)=c GL (A) c { }⊂ 1 of elements of reduced norm c is a torsor under the special linear group G = SL1(A) which is semisimple and simply connected. If cd(k) 2, we thus expect that this ≤ G-torsor is trivial, i.e., X (k) = /0.Applying this to all c k , we thus expect that c ∈ × the reduced norm map A k is surjective. × → × For imaginary number fields, the surjectivity of reduced norms goes back to Eich- ler in 1938 (see [40, 5.4]). For function fields of complex surfaces, this follows § from the Tsen-Lang theorem because the reduced norm is a homogeneous form of degree deg(A) in deg(A)2-indeterminates [54, II.4.5]. In the general case, the surjec- Philippe Gille DMA, UMR 8553 du CNRS, Ecole normale superieure,´ 45 rue d’Ulm, F-75005 Paris, France e-mail: [email protected] 37 38 Philippe Gille tivity of reduced norms is due to Merkurjev-Suslin in 1981 [60, Th. -
Historical Background
Chapter 0 Historical Background The theory of composition of quadratic forms over fields had its start in the 19th century with the search for n-square identities of the type 2 + 2 +···+ 2 · 2 + 2 +···+ 2 = 2 + 2 +···+ 2 (x1 x2 xn) (y1 y2 yn) z1 z2 zn where X = (x1,x2,...,xn) and Y = (y1,y2,...,yn) are systems of indeterminates and each zk = zk(X, Y ) is a bilinear form in X and Y . For example when n = 2 there is the ancient identity 2 + 2 · 2 + 2 = + 2 + − 2 (x1 x2 ) (y1 y2 ) (x1y1 x2y2) (x1y2 x2y1) . In this example z1 = x1y1 + x2y2 and z2 = x1y2 − x2y1 are bilinear forms in X, Y with integer coefficients. This formula for n = 2 can be interpreted as the “law of moduli” for complex numbers: |α|·|β|=|αβ| where α = x1 −ix2 and β = y1 +iy2. A similar 4-square identity was found by Euler (1748) in his attempt to prove Fermat’s conjecture that every positive integer is a sum of four integer squares. This identity is often attributed to Lagrange, who used it (1770) in his proof of that conjec- ture of Fermat. Here is Euler’s formula, in our notation: 2 + 2 + 2 + 2 · 2 + 2 + 2 + 2 = 2 + 2 + 2 + 2 (x1 x2 x3 x4 ) (y1 y2 y3 y4 ) z1 z2 z3 z4 where z1 = x1y1 + x2y2 + x3y3 + x4y4 z2 = x1y2 − x2y1 + x3y4 − x4y3 z3 = x1y3 − x2y4 − x3y1 + x4y2 z4 = x1y4 + x2y3 − x3y2 − x4y1. After Hamilton’s discovery of the quaternions (1843) this 4-square formula was interpreted as the law of moduli for quaternions. -
Cohomological Dimension of the Braid and Mapping Class Groups of Surfaces Daciberg Lima Gonçalves, John Guaschi, Miguel Maldonado
Embeddings and the (virtual) cohomological dimension of the braid and mapping class groups of surfaces Daciberg Lima Gonçalves, John Guaschi, Miguel Maldonado To cite this version: Daciberg Lima Gonçalves, John Guaschi, Miguel Maldonado. Embeddings and the (virtual) cohomo- logical dimension of the braid and mapping class groups of surfaces. Confluentes Mathematici, Institut Camille Jordan et Unité de Mathématiques Pures et Appliquées, 2018, 10 (1), pp.41-61. hal-01377681 HAL Id: hal-01377681 https://hal.archives-ouvertes.fr/hal-01377681 Submitted on 11 Oct 2016 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. EMBEDDINGS AND THE (VIRTUAL) COHOMOLOGICAL DIMENSION OF THE BRAID AND MAPPING CLASS GROUPS OF SURFACES DACIBERG LIMA GONC¸ALVES, JOHN GUASCHI, AND MIGUEL MALDONADO Abstract. In this paper, we make use of the relations between the braid and mapping class groups of a compact, connected, non-orientable surface N without boundary and those of its orientable double covering S to study embeddings of these groups and their (virtual) cohomological dimensions. We first generalise results of [4, 14] to show that the mapping class group MCG(N; k) of N relative to a k-point subset embeds in the mapping class group MCG(S; 2k) of S relative to a 2k-point subset. -
Isotropy of Quadratic Spaces in Finite and Infinite Dimension
Documenta Math. 473 Isotropy of Quadratic Spaces in Finite and Infinite Dimension Karim Johannes Becher, Detlev W. Hoffmann Received: October 6, 2006 Revised: September 13, 2007 Communicated by Alexander Merkurjev Abstract. In the late 1970s, Herbert Gross asked whether there exist fields admitting anisotropic quadratic spaces of arbitrarily large finite dimensions but none of infinite dimension. We construct exam- ples of such fields and also discuss related problems in the theory of central simple algebras and in Milnor K-theory. 2000 Mathematics Subject Classification: 11E04, 11E81, 12D15, 12E15, 12F20, 12G05, 12G10, 16K20, 19D45 Keywords and Phrases: quadratic form, isotropy, infinite-dimensional quadratic space, u-invariant, function field of a quadric, totally indef- inite form, real field, division algebra, quaternion algebra, symbol al- gebra, Galois cohomology, cohomological dimension, Milnor K-theory 1 Introduction A quadratic space over a field F is a pair (V, q) of a vector space V over F together with a map q : V F such that −→ q(λx) = λ2q(x) for all λ F , x V , and • ∈ ∈ the map bq : V V F defined by bq(x,y) = q(x + y) q(x) q(y) • (x,y V ) is F -bilinear.× −→ − − ∈ If dim V = n < , one may identify (after fixing a basis of V ) the quadratic space (V, q) with∞ a form (a homogeneous polynomial) of degree 2 in n variables. Via this identification, a finite-dimensional quadratic space over F will also be referred to as quadratic form over F . Recall that a quadratic space (V, q) is Documenta Mathematica 12 (2007) 473–504 474 Karim Johannes Becher, Detlev W. -
Quadratic Form - Wikipedia, the Free Encyclopedia
Quadratic form - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Quadratic_form Quadratic form From Wikipedia, the free encyclopedia In mathematics, a quadratic form is a homogeneous polynomial of degree two in a number of variables. For example, is a quadratic form in the variables x and y. Quadratic forms occupy a central place in various branches of mathematics, including number theory, linear algebra, group theory (orthogonal group), differential geometry (Riemannian metric), differential topology (intersection forms of four-manifolds), and Lie theory (the Killing form). Contents 1 Introduction 2 History 3 Real quadratic forms 4 Definitions 4.1 Quadratic spaces 4.2 Further definitions 5 Equivalence of forms 6 Geometric meaning 7 Integral quadratic forms 7.1 Historical use 7.2 Universal quadratic forms 8 See also 9 Notes 10 References 11 External links Introduction Quadratic forms are homogeneous quadratic polynomials in n variables. In the cases of one, two, and three variables they are called unary, binary, and ternary and have the following explicit form: where a,…,f are the coefficients.[1] Note that quadratic functions, such as ax2 + bx + c in the one variable case, are not quadratic forms, as they are typically not homogeneous (unless b and c are both 0). The theory of quadratic forms and methods used in their study depend in a large measure on the nature of the 1 of 8 27/03/2013 12:41 Quadratic form - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Quadratic_form coefficients, which may be real or complex numbers, rational numbers, or integers. In linear algebra, analytic geometry, and in the majority of applications of quadratic forms, the coefficients are real or complex numbers. -
Algebraic Groups I. Homework 8 1. Let a Be a Central Simple Algebra Over a field K, T a K-Torus in A×
Algebraic Groups I. Homework 8 1. Let A be a central simple algebra over a field k, T a k-torus in A×. (i) Adapt Exercise 5 in HW5 to make an ´etalecommutative k-subalgebra AT ⊆ A such that (AT )ks is × generated by T (ks), and establish a bijection between the sets of maximal k-tori in A and maximal ´etale commutative k-subalgebras of A. Deduce that SL(A) is k-anisotropic if and only if A is a division algebra. (ii) For an ´etalecommutative k-subalgebra C ⊆ A, prove ZA(C) is a semisimple k-algebra with center C. × (iv) If T is maximal as a k-split subtorus of A prove T is the k-group of units in AT and that the (central!) simple factors Bi of BT := ZA(AT ) are division algebras. (v) Fix A ' EndD(V ) for a right module V over a central division algebra D, so V is a left A-module and Q × V = Vi with nonzero left Bi-modules Vi. If T is maximal as a k-split torus in A , prove Vi has rank 1 × × over Bi and D, so Bi ' D. Using D-bases, deduce that all maximal k-split tori in A are A (k)-conjugate. 2. For a torus T over a local field k (allow R, C), prove T is k-anisotropic if and only if T (k) is compact. 3. Let Y be a smooth separated k-scheme locally of finite type, and T a k-torus with a left action on Y . -
THE DIMENSION of the TORELLI GROUP 1. Introduction Let S Be A
JOURNAL OF THE AMERICAN MATHEMATICAL SOCIETY Volume 23, Number 1, January 2010, Pages 61–105 S 0894-0347(09)00643-2 Article electronically published on July 10, 2009 THE DIMENSION OF THE TORELLI GROUP MLADEN BESTVINA, KAI-UWE BUX, AND DAN MARGALIT 1. Introduction Let S be a surface (unless specified otherwise, we take all surfaces to be con- nected, orientable, and of finite type). Let Mod(S)bethemapping class group of S, + + defined as π0(Homeo (S)), where Homeo (S) is the group of orientation-preserving homeomorphisms of S.TheTorelli group I(S) is the kernel of the natural action of Mod(S)onH1(S, Z). When S is Sg, the closed surface of genus g, this action is symplectic (it preserves the algebraic intersection number), and it is a classical fact that Mod(Sg) surjects onto Sp(2g, Z). All of this information is encoded in the following short exact sequence: 1 −→I(Sg) −→ Mod(Sg) −→ Sp(2g, Z) −→ 1. Cohomological dimension. For a group G,wedenotebycd(G)itscohomological dimension, which is the supremum over all n so that there exists a G-module M with Hn(G, M) =0. Theorem A. For g ≥ 2, we have cd(I(Sg)) = 3g − 5. ∼ Since Mod(S0)=1,wehaveI(S0) = 1. Also, it is a classical fact that Mod(S1) = Sp(2, Z)=SL(2, Z), and so I(S1)isalsotrivial. The lower bound for Theorem A was already given by Mess [36] in an unpublished paper from 1990. Theorem 1.1 (Mess). For g ≥ 2, we have cd(I(Sg)) ≥ 3g − 5. -
Subfields of Central Simple Algebras
The Brauer Group Subfields of Algebras Subfields of Central Simple Algebras Patrick K. McFaddin University of Georgia Feb. 24, 2016 Patrick K. McFaddin University of Georgia Subfields of Central Simple Algebras The Brauer Group Subfields of Algebras Introduction Central simple algebras and the Brauer group have been well studied over the past century and have seen applications to class field theory, algebraic geometry, and physics. Since higher K-theory defined in '72, the theory of algebraic cycles have been utilized to study geometric objects associated to central simple algebras (with involution). This new machinery has provided a functorial viewpoint in which to study questions of arithmetic. Patrick K. McFaddin University of Georgia Subfields of Central Simple Algebras The Brauer Group Subfields of Algebras Central Simple Algebras Let F be a field. An F -algebra A is a ring with identity 1 such that A is an F -vector space and α(ab) = (αa)b = a(αb) for all α 2 F and a; b 2 A. The center of an algebra A is Z(A) = fa 2 A j ab = ba for every b 2 Ag: Definition A central simple algebra over F is an F -algebra whose only two-sided ideals are (0) and (1) and whose center is precisely F . Examples An F -central division algebra, i.e., an algebra in which every element has a multiplicative inverse. A matrix algebra Mn(F ) Patrick K. McFaddin University of Georgia Subfields of Central Simple Algebras The Brauer Group Subfields of Algebras Why Central Simple Algebras? Central simple algebras are a natural generalization of matrix algebras. -
Partial Euler Characteristic, Normal Generations and the Stable D (2
Partial Euler Characteristic, Normal Generations and the stable D(2) problem Feng Ji and Shengkui Ye October 10, 2018 Abstract We study the interplay among Wall’s D(2) problem, normal generation conjecture (the Wiegold Conjecture) of perfect groups and Swan’s problem on partial Euler characteristic and deficiency of groups. In particular, for a 3-dimensional complex X of cohomological dimension 2 with finite fundamental group, assuming the Wiegold conjecture holds, we prove that X is homotopy equivalent to a finite 2-complex after wedging a copy of 2 sphere S . 1 Introduction In this article, we study several classical problems in low-dimensional homotopy theory and group theory, focusing on the interplay among these problems. Let us first recall the following Swan’s problem. Let G be a group and Z G the group ring. Swan [16] defines the partial Euler characteristic µn(G) as follows. Let F be a resolution ···→ F2 → F1 → F0 → Z → 0 of the trivial ZG-module Z, in which each Fi is ZG-free on fi generators. For an integer n ≥ 0, if f0,f1,f2, ··· ,fn arXiv:1503.01987v2 [math.AT] 16 Jan 2018 are finite, define n µn(F )= fn − fn−1 + fn−2 − · · · + (−1) f0. If there exists a resolution F such that µn(F ) is defined, we let µn(G) be the infimum of µn(F ) over all such resolutions F. We call the truncated free resolu- tion Fn →···→ F1 → F0 → Z → 0 an algebraic n-complex if each Fi is finitely generated as a ZG-module (following the terminology of Johnson [8]). -
Orthogonal Symmetries and Clifford Algebras 11
ORTHOGONAL SYMMETRIES AND CLIFFORD ALGEBRAS M. G. MAHMOUDI Abstract. Involutions of the Clifford algebra of a quadratic space induced by orthogonal symmetries are investigated. 2000 Mathematics Subject Classification: 16W10, 11E39 Key words: Orthogonal symmetry, reflection, Clifford algebra, Involution, qua- dratic form, even Clifford algebra, multiquaternion algebra, universal property of Clifford algebra, Clifford map 1. Introduction Clifford algebra is one of the important algebraic structures which can be associ- ated to a quadratic form. These algebras are among the most fascinating algebraic structures. Not only they have many applications in algebra and other branches of Mathematics, but also they have wide applications beyond Mathematics, e.g., Physics, Computer Science and Engineering [6], [18], [14], [15]. A detailed historical account of Clifford algebras from their genesis can be found in [21]. See also [7] and [14] for an interesting brief historical account of Clifford algebras. Many familiar algebras can be regarded as special cases of Clifford algebras. For example the algebra of Complex numbers is isomorphic to the Clifford algebra of any one-dimensional negative definite quadratic form over R. The algebra of Hamilton quaternions is isomorphic to the Clifford algebras of a two-dimensional negative definite quadratic form over R. More generally, as shown by D. Lewis in [13, Proposition 1], a multiquaternion algebra, i.e., an algebra like Q1 Qn ⊗···⊗ff where Qi is a quaternion algebra over a field K, can be regarded as the Cli ord algebra of a suitable nondegenerate quadratic form q over the base field K. In [13], such a form q is also explicitly constructed.