DOCSLIB.ORG

Differential Geometry

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Differential Geometry DIFFERENTIAL GEOMETRY MARC BURGER STEPHAN TORNIER Abstract. These are notes of the course Differential Geometry I held at ETH Zurich in 2015. Disclaimer. This is a preliminary version. Please report any typos, mistakes, comments etc. to [email protected]. Acknowledgements. Thanks to those who pointed out typos and mistakes as these notes were written, in particular J. Allemann, H. Benner, P. Gantner, A. Hauswirth, S. Imfeld, C. Macho, P. Poklukar and H. Wu. Contents Introduction................................... 1 1. Differential Manifolds and Differentiable Maps . ..... 2 2. Tangent spaces, Differential and Whitney’s Embedding Theorem .. 21 3. Differential Forms and Integration on Manifolds . ..... 31 4. DeRhamCohomology ........................... 54 5. DeRham’sTheorem ............................ 65 References .................................... 71 Introduction Differential geometry is a synthesis of three different subjects: Analysis in Rn, topology and multilinear algebra. It precisely defines a class of “spaces” on which one can do analysis, termed differential manifolds. These spaces as well as the associated notion of differentiable functions are the central concept of this course. Differential manifolds look locally like Rn but are globally much less boring. Examples are the sphere as well as surfaces with holes: Sphere Torus Surface with two holes An example of what we mean by “do analysis” is the following: If D is a region in R2 contoured by a curve σ : [0, 1] R2 and L,M : R2 R are reasonably smooth functions, then by Green’s formula:→ → ∂M ∂L L(x, y) dx + M(x, y) dy = dx dy. ∂x − ∂y Ic ZZD Note that whereas the left-hand side of the equation only takes into account the values of L and M on c it yet remembers something about D, namely the right-hand side. Green’s formula, the divergence theorem and other well-known formulas are Date: June 3, 2016. 1 2 MARCBURGERSTEPHANTORNIER all incarnations of Stokes’ Theorem which is best understood in the framework of manifolds. In this context we will have to make precise what object makes sense to be integrated over a manifold: differential forms. Stokes’ Theorem will also be used to define invariants that can e.g. tell the above surfaces apart in the sense that one cannot be deformed into the other without tearing it apart. Differential geometry forms a basis for many other subjects. Clearly Riemannian geometry is one of them. However, so are Lie groups and physics. The relationship between differential geometry in algebraic geometry is a special one. In a way, notions of one of the two fields echo in the other. For instance, highly abstract algebraic geometric concepts are often easier to visualize in differential geometry. References for this course include Boothby’s [Boo03] which is readable by stu- dents, Barden’s and Thomas’ [BT03] which will in particular be used for differential forms and Milnor’s classic [Mil97] on which our section on Brouwer’s fixed theorem will be based. 1. Differential Manifolds and Differentiable Maps 1.1. Differential Manifolds: Definitions and Examples. Differential mani- folds were first studied by Riemann in 1854 and later on by Poincaré. However, they were still thinking about manifolds being imbedded in some euclidean space and lacked a precise definition. Nevertheless, Stokes’ theorem and notions like cur- vature were already around. The first precise definition, however, was given in 1913 by Weyl at ETH, see [RW13]. 1.2. Differential Manifolds: Definitions and Examples. The first step to- wards defining differential manifolds is to introduce topological manifolds. Definition 1.1. An n-dimensional topological manifold is a topological space which is (i) Hausdorff (T2), (ii) second countable, and (iii) locally homeomorphic to Rn. Clearly, the last condition of 1.1 is the crucial one. Recall that a topological space X is Hausdorff if for every pair of points x, y X with x = y there are open sets U, V X containing x and y respectively such∈ that U V 6 = . Moreover, X is second⊆ countable if it its topology admits a countable basis∩ =∅ U n N , B { n | ∈ } i.e. any open set U X can be written as U = Un. Eventually, X is locally ⊆ Un⊆U homeomorphic to Rn if every point in M admits an open neighbourhood which is n S homeomorphic to an open subset of R . We are going to discuss many examples and non-examples of differential mani- folds. Therefore our list of examples and non-examples of topological manifolds is rather short. Example 1.2. (i) Let M be a countable discrete space. Then M is a zero-dimensional topo- logical manifold. The converse is true as well. However, the classification of n-dimensional manifolds is much harder in larger n. (ii) The circle S1 R2 with the induced topology is clearly a topological mani- fold of dimension⊆ one. Every point p admits a neighbourhood that is home- omorphic to an interval: b p DIFFERENTIAL GEOMETRY 3 (iii) The real line R is a one-dimensional topological manifold as well. This is tautological. If one restricts oneself to connected, one-dimensional topological manifolds then S1 and R are in fact the only examples up to homeomorphism. (iv) In dimension two, the situation is already so rich that it defies any reason- able classification. Examples are, as before, the family of surfaces with g holes (g N0): These surfaces are all compact and connected and in fact a classification∈ of compact, connected, two-dimensional topological mani- folds is managable. To this end we will later on introduce the notion of orientability to distinguish between orientable examples as above and non- orientable examples like projective space. To get an idea of the wealth of general (connected) two-dimensional topological manifolds, note that the complement in R2 of a Cantor set is an example. Non-Example 1.3. (i) Consider M := [0, 1] R with the induced topology. Every interior point of M satisfies the third⊂ requirement of the definition of a topological man- ifold for n = 1 but the points 0, 1 M do not. For instance, a typical neighbourhood of 0 M is given by∈U = [0,ε) for some 0 <ε< 1. Sup- pose ϕ : U V ∈R is a homeomorphism onto an open subset V of R. Note U 0 →is connected,⊆ but V ϕ(0) is not. This contradicts ϕ being continuous.\{ } \{ } (ii) The set M := [0, 1]2 is not a (two-dimensional) topological manifold either. Here one may argue using the fundamental group instead of connectedness. Both non-examples above are manifolds with boundary though, as defined later. (iii) Let M = Q R with the induced topology. Then every open set of Q is countable and⊂ hence cannot be in bijection with an open subset of any Rn. Remark 1.4. In order to show that the dimension of a topological manifold is well-defined one has to show that if U Rn and V Rm are non-empty and homeomorphic then n = m. If n =1 or m⊆=1 one may⊆ argue in the above fashion using connectedness. The other cases require e.g. some basic homology theory. If, however, one asks for U and V to be C1-diffeomorphic then basic linear algebra applied to the derivative readily implies n = m. This argument is going to apply in the context of differential manifolds. The next definition constitutes the next important step towards the definition of differential manifolds. Definition 1.5. Let M be an n-dimensional topological manifold. A chart on M is a pair (U, ϕ) consisting of an open subset U M and a homoeomorphism ϕ : U ϕ(U) =: V Rn. The subset U is the coordinate⊆ neighbourhood and V is the coordinate→ space.⊆ We are now going to examine the case in which two charts intersect, see below. 4 MARCBURGERSTEPHANTORNIER Uα Uβ ϕα ϕβ ϕα(Uα) θβα ϕβ(Uβ) θαβ The maps θ := ϕ (ϕ )−1 : ϕ (U U ) ϕ (U U ), βα β ◦ α|Uα∩Uβ α α ∩ β → β α ∩ β θ := ϕ (ϕ )−1 : ϕ (U U ) ϕ (U U ), αβ α ◦ β |Uβ ∩Uα β α ∩ β → α α ∩ β are coordinate transformations or change of charts. They are homeomorphisms n −1 between open subsets of R as θβα = θαβ. Definition 1.6. Let M be a topological manifold. A C0-atlas on M is a collection of charts = (U , ϕ ) chart on M α A such that AU = M. A { α α | ∈ } α∈A α Note that our definition perfectly resembles real-life atlaS ses, introduced by Mer- cator in 1585. By Definition 1.1, every topological manifold admits a C0-atlas. As a next step towards the definition of differential manifolds we now introduce smooth atlases. Definition 1.7. Let be a C0-atlas. Then is a Ck-atlas if all coordinate trans- formations betweenA members of are Ck-maps.A A C∞-atlas is smooth. A Recall that a map f : U Rm from an open subset U Rn to Rm given by x (f (x),...,f (x)) where→f := π f for all i 1,...,m⊆ is Ck if all partial 7→ 1 m i i ◦ ∈{ } derivatives of fi (i 1,...,m ) up to order k exist and are continuous. Also, note that in a Ck-atlas all∈{ coordinate} transformations are in fact Ck-diffeomorphisms by the above, in contrast to the fact that there is a smooth homeomorphism f : R R which is not a diffeomorphism, for instance f : x x3. In particular, this→ map cannot occur as a coordinate transformation in any7→ smooth atlas. Example 1.8. We now give examples of topological manifolds with smooth atlases. (i) Let M := U be an open subset of Rn and = (U, id) .
Load more

Recommended publications
  • Multilinear Algebra and Applications July 15, 2014
    Multilinear Algebra and Applications July 15, 2014. Contents Chapter 1. Introduction 1 Chapter 2. Review of Linear Algebra 5 2.1. Vector Spaces and Subspaces 5 2.2. Bases 7 2.3. The Einstein convention 10 2.3.1. Change of bases, revisited 12 2.3.2. The Kronecker delta symbol 13 2.4. Linear Transformations 14 2.4.1. Similar matrices 18 2.5. Eigenbases 19 Chapter 3. Multilinear Forms 23 3.1. Linear Forms 23 3.1.1. Definition, Examples, Dual and Dual Basis 23 3.1.2. Transformation of Linear Forms under a Change of Basis 26 3.2. Bilinear Forms 30 3.2.1. Definition, Examples and Basis 30 3.2.2. Tensor product of two linear forms on V 32 3.2.3. Transformation of Bilinear Forms under a Change of Basis 33 3.3. Multilinear forms 34 3.4. Examples 35 3.4.1. A Bilinear Form 35 3.4.2. A Trilinear Form 36 3.5. Basic Operation on Multilinear Forms 37 Chapter 4. Inner Products 39 4.1. Definitions and First Properties 39 4.1.1. Correspondence Between Inner Products and Symmetric Positive Definite Matrices 40 4.1.1.1. From Inner Products to Symmetric Positive Definite Matrices 42 4.1.1.2. From Symmetric Positive Definite Matrices to Inner Products 42 4.1.2. Orthonormal Basis 42 4.2. Reciprocal Basis 46 4.2.1. Properties of Reciprocal Bases 48 4.2.2. Change of basis from a basis to its reciprocal basis g 50 B B III IV CONTENTS 4.2.3.
    [Show full text]
  • Multilinear Algebra
    Appendix A Multilinear Algebra This chapter presents concepts from multilinear algebra based on the basic properties of finite dimensional vector spaces and linear maps. The primary aim of the chapter is to give a concise introduction to alternating tensors which are necessary to define differential forms on manifolds. Many of the stated definitions and propositions can be found in Lee [1], Chaps. 11, 12 and 14. Some definitions and propositions are complemented by short and simple examples. First, in Sect. A.1 dual and bidual vector spaces are discussed. Subsequently, in Sects. A.2–A.4, tensors and alternating tensors together with operations such as the tensor and wedge product are introduced. Lastly, in Sect. A.5, the concepts which are necessary to introduce the wedge product are summarized in eight steps. A.1 The Dual Space Let V be a real vector space of finite dimension dim V = n.Let(e1,...,en) be a basis of V . Then every v ∈ V can be uniquely represented as a linear combination i v = v ei , (A.1) where summation convention over repeated indices is applied. The coefficients vi ∈ R arereferredtoascomponents of the vector v. Throughout the whole chapter, only finite dimensional real vector spaces, typically denoted by V , are treated. When not stated differently, summation convention is applied. Definition A.1 (Dual Space)Thedual space of V is the set of real-valued linear functionals ∗ V := {ω : V → R : ω linear} . (A.2) The elements of the dual space V ∗ are called linear forms on V . © Springer International Publishing Switzerland 2015 123 S.R.
    [Show full text]
  • Vector Calculus and Differential Forms
    Vector Calculus and Differential Forms John Terilla Math 208 Spring 2015 1 Vector fields Definition 1. Let U be an open subest of Rn.A tangent vector in U is a pair (p; v) 2 U × Rn: We think of (p; v) as consisting of a vector v 2 Rn lying at the point p 2 U. Often, we denote a tangent vector by vp instead of (p; v). For p 2 U, the set of all tangent vectors at p is denoted by Tp(U). The set of all tangent vectors in U is denoted T (U) For a fixed point p 2 U, the set Tp(U) is a vector space with addition and scalar multiplication defined by vp + wp = (v + w)p and αvp = (αv)p: Note n that as a vector space, Tp(U) is isomorphic to R . Definition 2. A vector field on U is a function X : U ! T (Rn) satisfying X(p) 2 Tp(U). Remark 1. Notice that any function f : U ! Rn defines a vector field X by the rule X(p) = f(p)p: Denote the set of vector fields on U by Vect(U). Note that Vect(U) is a vector space with addition and scalar multiplication defined pointwise (which makes sense since Tp(U) is a vector space): (X + Y )(p) := X(p) + Y (p) and (αX)(p) = α(X(p)): Definition 3. Denote the set of functions on the set U by Fun(U) = ff : U ! Rg. Let C1(U) be the subset of Fun(U) consisting of functions with continuous derivatives and let C1(U) be the subset of Fun(U) consisting of smooth functions, i.e., infinitely differentiable functions.
    [Show full text]
  • Ring (Mathematics) 1 Ring (Mathematics)
    Ring (mathematics) 1 Ring (mathematics) In mathematics, a ring is an algebraic structure consisting of a set together with two binary operations usually called addition and multiplication, where the set is an abelian group under addition (called the additive group of the ring) and a monoid under multiplication such that multiplication distributes over addition.a[›] In other words the ring axioms require that addition is commutative, addition and multiplication are associative, multiplication distributes over addition, each element in the set has an additive inverse, and there exists an additive identity. One of the most common examples of a ring is the set of integers endowed with its natural operations of addition and multiplication. Certain variations of the definition of a ring are sometimes employed, and these are outlined later in the article. Polynomials, represented here by curves, form a ring under addition The branch of mathematics that studies rings is known and multiplication. as ring theory. Ring theorists study properties common to both familiar mathematical structures such as integers and polynomials, and to the many less well-known mathematical structures that also satisfy the axioms of ring theory. The ubiquity of rings makes them a central organizing principle of contemporary mathematics.[1] Ring theory may be used to understand fundamental physical laws, such as those underlying special relativity and symmetry phenomena in molecular chemistry. The concept of a ring first arose from attempts to prove Fermat's last theorem, starting with Richard Dedekind in the 1880s. After contributions from other fields, mainly number theory, the ring notion was generalized and firmly established during the 1920s by Emmy Noether and Wolfgang Krull.[2] Modern ring theory—a very active mathematical discipline—studies rings in their own right.
    [Show full text]
  • Chapter IX. Tensors and Multilinear Forms
    Notes c F.P. Greenleaf and S. Marques 2006-2016 LAII-s16-quadforms.tex version 4/25/2016 Chapter IX. Tensors and Multilinear Forms. IX.1. Basic Definitions and Examples. 1.1. Definition. A bilinear form is a map B : V V C that is linear in each entry when the other entry is held fixed, so that × → B(αx, y) = αB(x, y)= B(x, αy) B(x + x ,y) = B(x ,y)+ B(x ,y) for all α F, x V, y V 1 2 1 2 ∈ k ∈ k ∈ B(x, y1 + y2) = B(x, y1)+ B(x, y2) (This of course forces B(x, y)=0 if either input is zero.) We say B is symmetric if B(x, y)= B(y, x), for all x, y and antisymmetric if B(x, y)= B(y, x). Similarly a multilinear form (aka a k-linear form , or a tensor− of rank k) is a map B : V V F that is linear in each entry when the other entries are held fixed. ×···×(0,k) → We write V = V ∗ . V ∗ for the set of k-linear forms. The reason we use V ∗ here rather than V , and⊗ the⊗ rationale for the “tensor product” notation, will gradually become clear. The set V ∗ V ∗ of bilinear forms on V becomes a vector space over F if we define ⊗ 1. Zero element: B(x, y) = 0 for all x, y V ; ∈ 2. Scalar multiple: (αB)(x, y)= αB(x, y), for α F and x, y V ; ∈ ∈ 3. Addition: (B + B )(x, y)= B (x, y)+ B (x, y), for x, y V .
    [Show full text]
  • Tensor Complexes: Multilinear Free Resolutions Constructed from Higher Tensors
    Tensor complexes: Multilinear free resolutions constructed from higher tensors C. Berkesch, D. Erman, M. Kummini and S. V. Sam REPORT No. 7, 2010/2011, spring ISSN 1103-467X ISRN IML-R- -7-10/11- -SE+spring TENSOR COMPLEXES: MULTILINEAR FREE RESOLUTIONS CONSTRUCTED FROM HIGHER TENSORS CHRISTINE BERKESCH, DANIEL ERMAN, MANOJ KUMMINI, AND STEVEN V SAM Abstract. The most fundamental complexes of free modules over a commutative ring are the Koszul complex, which is constructed from a vector (i.e., a 1-tensor), and the Eagon– Northcott and Buchsbaum–Rim complexes, which are constructed from a matrix (i.e., a 2-tensor). The subject of this paper is a multilinear analogue of these complexes, which we construct from an arbitrary higher tensor. Our construction provides detailed new examples of minimal free resolutions, as well as a unifying view on a wide variety of complexes including: the Eagon–Northcott, Buchsbaum– Rim and similar complexes, the Eisenbud–Schreyer pure resolutions, and the complexes used by Gelfand–Kapranov–Zelevinsky and Weyman to compute hyperdeterminants. In addition, we provide applications to the study of pure resolutions and Boij–S¨oderberg theory, including the construction of infinitely many new families of pure resolutions, and the first explicit description of the differentials of the Eisenbud–Schreyer pure resolutions. 1. Introduction In commutative algebra, the Koszul complex is the mother of all complexes. David Eisenbud The most fundamental complex of free modules over a commutative ring R is the Koszul a complex, which is constructed from a vector (i.e., a 1-tensor) f = (f1, . , fa) R . The next most fundamental complexes are likely the Eagon–Northcott and Buchsbaum–Rim∈ complexes, which are constructed from a matrix (i.e., a 2-tensor) ψ Ra Rb.
    [Show full text]
  • Cross Products, Automorphisms, and Gradings 3
    CROSS PRODUCTS, AUTOMORPHISMS, AND GRADINGS ALBERTO DAZA-GARC´IA, ALBERTO ELDUQUE, AND LIMING TANG Abstract. The affine group schemes of automorphisms of the multilinear r- fold cross products on finite-dimensional vectors spaces over fields of character- istic not two are determined. Gradings by abelian groups on these structures, that correspond to morphisms from diagonalizable group schemes into these group schemes of automorphisms, are completely classified, up to isomorphism. 1. Introduction Eckmann [Eck43] defined a vector cross product on an n-dimensional real vector space V , endowed with a (positive definite) inner product b(u, v), to be a continuous map X : V r −→ V (1 ≤ r ≤ n) satisfying the following axioms: b X(v1,...,vr), vi =0, 1 ≤ i ≤ r, (1.1) bX(v1,...,vr),X(v1,...,vr) = det b(vi, vj ) , (1.2) There are very few possibilities. Theorem 1.1 ([Eck43, Whi63]). A vector cross product exists in precisely the following cases: • n is even, r =1, • n ≥ 3, r = n − 1, • n =7, r =2, • n =8, r =3. Multilinear vector cross products X on vector spaces V over arbitrary fields of characteristic not two, relative to a nondegenerate symmetric bilinear form b(u, v), were classified by Brown and Gray [BG67]. These are the multilinear maps X : V r → V (1 ≤ r ≤ n) satisfying (1.1) and (1.2). The possible pairs (n, r) are again those in Theorem 1.1. The exceptional cases: (n, r) = (7, 2) and (8, 3), are intimately related to the arXiv:2006.10324v1 [math.RT] 18 Jun 2020 octonion, or Cayley, algebras.
    [Show full text]
  • Singularities of Hyperdeterminants Annales De L’Institut Fourier, Tome 46, No 3 (1996), P
    ANNALES DE L’INSTITUT FOURIER JERZY WEYMAN ANDREI ZELEVINSKY Singularities of hyperdeterminants Annales de l’institut Fourier, tome 46, no 3 (1996), p. 591-644 <http://www.numdam.org/item?id=AIF_1996__46_3_591_0> © Annales de l’institut Fourier, 1996, tous droits réservés. L’accès aux archives de la revue « Annales de l’institut Fourier » (http://annalif.ujf-grenoble.fr/) implique l’accord avec les conditions gé- nérales d’utilisation (http://www.numdam.org/conditions). Toute utilisa- tion commerciale ou impression systématique est constitutive d’une in- fraction pénale. Toute copie ou impression de ce fichier doit conte- nir la présente mention de copyright. Article numérisé dans le cadre du programme Numérisation de documents anciens mathématiques http://www.numdam.org/ Ann. Inst. Fourier, Grenoble 46, 3 (1996), 591-644 SINGULARITIES OF HYPERDETERMINANTS by J. WEYMAN (1) and A. ZELEVINSKY (2) Contents. 0. Introduction 1. Symmetric matrices with diagonal lacunae 2. Cusp type singularities 3. Eliminating special node components 4. The generic node component 5. Exceptional cases: the zoo of three- and four-dimensional matrices 6. Decomposition of the singular locus into cusp and node parts 7. Multi-dimensional "diagonal" matrices and the Vandermonde matrix 0. Introduction. In this paper we continue the study of hyperdeterminants recently undertaken in [4], [5], [12]. The hyperdeterminants are analogs of deter- minants for multi-dimensional "matrices". Their study was initiated by (1) Partially supported by the NSF (DMS-9102432). (2) Partially supported by the NSF (DMS- 930424 7). Key words: Hyperdeterminant - Singular locus - Cusp type singularities - Node type singularities - Projectively dual variety - Segre embedding. Math. classification: 14B05.
    [Show full text]
  • 4 Exterior Algebra
    4 Exterior algebra 4.1 Lines and 2-vectors The time has come now to develop some new linear algebra in order to handle the space of lines in a projective space P (V ). In the projective plane we have seen that duality can deal with this but lines in higher dimensional spaces behave differently. From the point of view of linear algebra we are looking at 2-dimensional vector sub- spaces U ⊂ V . To motivate what we shall do, consider how in Euclidean geometry we describe a 2-dimensional subspace of R3. We could describe it through its unit normal n, which is also parallel to u×v where u and v are linearly independent vectors in the space and u×v is the vector cross product. The vector product has the following properties: • u×v = −v×u • (λ1u1 + λ2u2)×v = λ1u1×v + λ2u2×v We shall generalize these properties to vectors in any vector space V – the difference is that the product will not be a vector in V , but will lie in another associated vector space. Definition 12 An alternating bilinear form on a vector space V is a map B : V × V → F such that • B(v, w) = −B(w, v) • B(λ1v1 + λ2v2, w) = λ1B(v1, w) + λ2B(v2, w) This is the skew-symmetric version of the symmetric bilinear forms we used to define quadrics. Given a basis {v1, . , vn}, B is uniquely determined by the skew symmetric matrix B(vi, vj). We can add alternating forms and multiply by scalars so they form a vector space, isomorphic to the space of skew-symmetric n × n matrices.
    [Show full text]
  • The Characteristic Masses of Niemeier Lattices Gaetan Chenevier
    The characteristic masses of Niemeier lattices Gaetan Chenevier To cite this version: Gaetan Chenevier. The characteristic masses of Niemeier lattices. Journal de Théorie des Nombres de Bordeaux, Société Arithmétique de Bordeaux, In press. hal-02915029 HAL Id: hal-02915029 https://hal.archives-ouvertes.fr/hal-02915029 Submitted on 13 Aug 2020 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. The characteristic masses of Niemeier lattices Gaëtan Chenevier∗ April 27, 2020 Abstract n Let L be an integral lattice in the Euclidean space R and W an n irreducible representation of the orthogonal group of R . We give an implemented algorithm computing the dimension of the subspace of invariants in W under the isometry group O(L) of L. A key step is the determination of the number of elements in O(L) having any given characteristic polynomial, a datum that we call the characteristic masses of L. As an application, we determine the characteristic masses of all the Niemeier lattices, and more generally of any even lattice of determinant ≤ 2 in dimension n ≤ 25. For Niemeier lattices, as a verification, we provide an alternative (human) computation of the characteristic masses.
    [Show full text]
  • MULTILINEAR ALGEBRA 1. Tensor and Symmetric Algebra Let K Be A
    CHAPTER XII MULTILINEAR ALGEBRA 1. Tensor and Symmetric Algebra Let k be a commutative ring. By a k-algebra, we mean a ring homomorphism φ : k → A such that each element of Im φ commutes with each element of A.(IfAis a ring, we define its center to be the subring Z(A)={a∈A|ax = xa, for all x ∈ A}. So this can also be abbreviated Im φ ⊆ Z(A).)) Such a k-algebra may be viewed as a k-module by defining ax = φ(a)x for a ∈ k and x ∈ A.Ifφ:k→A and ψ : k → B are k-algebras, a ring homomorphism f : A → B is called a k-algebra morphism if it is also a k-module homomorphism. We shall often speak loosely of a “k-algebra A” without mentioning the homomorphism φ explicitly. We may consider the forgetful functor which attaches to a k-algebra A the underlying k-module A. But to simplify the notation, we shall write A both for the ring and the associated k-module, with the context making clear which is intended. We want to construct a left adjoint to this functor. That will amount to the following: given a k-module M, we want to construct a k-algebra T (M) which contains M andsuchthateveryk-module homomorphism M → A where A is a k-algebra may be extended uniquely to a k-algebra morphism T (M) → A. In categorical terms, we will have an isomorphism of functors ∼ Homk−Alg (T (M),A)=Homk(M,A). To construct T (M), we form the so called tensor algebra.
    [Show full text]
  • Differential Geometry
    Differential Geometry Lectures by P.M.H. Wilson Notes by David Mehrle [email protected] Cambridge University Mathematical Tripos Part III Michaelmas 2015 Contents Lecture 1.............. 2 Lecture 13 ............. 35 Lecture 2.............. 4 Lecture 14 ............. 38 Lecture 3.............. 6 Lecture 15 ............. 41 Lecture 4.............. 9 Lecture 16 ............. 44 Lecture 5.............. 12 Lecture 17 ............. 48 Lecture 6.............. 15 Lecture 18 ............. 51 Lecture 7.............. 18 Lecture 19 ............. 54 Lecture 8.............. 20 Lecture 20 ............. 57 Lecture 9.............. 23 Lecture 21 ............. 60 Lecture 10 ............. 26 Lecture 22 ............. 64 Lecture 11 ............. 29 Lecture 23 ............. 68 Lecture 12 ............. 32 Lecture 24 ............. 70 Last updated April 1, 2016. 1 Lecture 1 8 October 2015 Administrative Stuff There are some Lecture Notes online. They have some stuff that we won’t cover. The best book is Spivak. Manifolds and Vector Spaces Smooth Manifolds Definition 1. If U Ă Rm and d : U Ñ R, we say that d is smooth or C8 if has continuous partial derivatives of all orders. Definition 2. A topological space X is called second countable if there exists a countable collection B of open subsets of X such that any open subset of X may be written as the union of sets of B. Definition 3. A Hausdorff, second countable topological space X is called a topological manifold of dimension d if each point has an open neighbor- hood (nbhd) homeomorphic to an open subset U of Rd by a homeomorphism f : U „ fpUq Ă Rd. The pair pU, fq of a homeomorphism and open subset of M is called a chart: given open subsets U and V of X with U X V ‰H, and charts pU, fUq and d d pV, fVq, with fU : U Ñ fpUq Ă R and fV : V Ñ fpVq Ă R , we have a ´1 homeomorphism fVU “ fV ˝ fU : fUpU X Vq Ñ fVpU X Vq of open subsets of Rd.
    [Show full text]