Homology, Brouwer's Fixed-Point Theorem, and Invariance of Domain
Total Page:16
File Type:pdf, Size:1020Kb
Load more
Recommended publications
-
The Boundary Manifold of a Complex Line Arrangement
Geometry & Topology Monographs 13 (2008) 105–146 105 The boundary manifold of a complex line arrangement DANIEL CCOHEN ALEXANDER ISUCIU We study the topology of the boundary manifold of a line arrangement in CP2 , with emphasis on the fundamental group G and associated invariants. We determine the Alexander polynomial .G/, and more generally, the twisted Alexander polynomial associated to the abelianization of G and an arbitrary complex representation. We give an explicit description of the unit ball in the Alexander norm, and use it to analyze certain Bieri–Neumann–Strebel invariants of G . From the Alexander polynomial, we also obtain a complete description of the first characteristic variety of G . Comparing this with the corresponding resonance variety of the cohomology ring of G enables us to characterize those arrangements for which the boundary manifold is formal. 32S22; 57M27 For Fred Cohen on the occasion of his sixtieth birthday 1 Introduction 1.1 The boundary manifold Let A be an arrangement of hyperplanes in the complex projective space CPm , m > 1. Denote by V S H the corresponding hypersurface, and by X CPm V its D H A D n complement. Among2 the origins of the topological study of arrangements are seminal results of Arnol’d[2] and Cohen[8], who independently computed the cohomology of the configuration space of n ordered points in C, the complement of the braid arrangement. The cohomology ring of the complement of an arbitrary arrangement A is by now well known. It is isomorphic to the Orlik–Solomon algebra of A, see Orlik and Terao[34] as a general reference. -
TOPOLOGY HW 8 55.1 Show That If a Is a Retract of B 2, Then Every
TOPOLOGY HW 8 CLAY SHONKWILER 55.1 Show that if A is a retract of B2, then every continuous map f : A → A has a fixed point. 2 Proof. Suppose r : B → A is a retraction. Thenr|A is the identity map on A. Let f : A → A be continuous and let i : A → B2 be the inclusion map. Then, since i, r and f are continuous, so is F = i ◦ f ◦ r. Furthermore, by the Brouwer fixed-point theorem, F has a fixed point x0. In other words, 2 x0 = F (x0) = (i ◦ f ◦ r)(x0) = i(f(r(x0))) ∈ A ⊆ B . Since x0 ∈ A, r(x0) = x0 and so x0F (x0) = i(f(r(x0))) = i(f(x0)) = f(x0) since i(x) = x for all x ∈ A. Hence, x0 is a fixed point of f. 55.4 Suppose that you are given the fact that for each n, there is no retraction f : Bn+1 → Sn. Prove the following: (a) The identity map i : Sn → Sn is not nulhomotopic. Proof. Suppose i is nulhomotopic. Let H : Sn × I → Sn be a homotopy between i and a constant map. Let π : Sn × I → Bn+1 be the map π(x, t) = (1 − t)x. π is certainly continuous and closed. To see that it is surjective, let x ∈ Bn+1. Then x x π , 1 − ||x|| = (1 − (1 − ||x||)) = x. ||x|| ||x|| Hence, since it is continuous, closed and surjective, π is a quotient map that collapses Sn×1 to 0 and is otherwise injective. Since H is constant on Sn×1, it induces, by the quotient map π, a continuous map k : Bn+1 → Sn that is an extension of i. -
Graph Manifolds and Taut Foliations Mark Brittenham1, Ramin Naimi, and Rachel Roberts2 Introduction
Graph manifolds and taut foliations 1 2 Mark Brittenham , Ramin Naimi, and Rachel Roberts UniversityofTexas at Austin Abstract. We examine the existence of foliations without Reeb comp onents, taut 1 1 foliations, and foliations with no S S -leaves, among graph manifolds. We show that each condition is strictly stronger than its predecessors, in the strongest p os- sible sense; there are manifolds admitting foliations of eachtyp e which do not admit foliations of the succeeding typ es. x0 Introduction Taut foliations have b een increasingly useful in understanding the top ology of 3-manifolds, thanks largely to the work of David Gabai [Ga1]. Many 3-manifolds admit taut foliations [Ro1],[De],[Na1], although some do not [Br1],[Cl]. To date, however, there are no adequate necessary or sucient conditions for a manifold to admit a taut foliation. This pap er seeks to add to this confusion. In this pap er we study the existence of taut foliations and various re nements, among graph manifolds. What we show is that there are many graph manifolds which admit foliations that are as re ned as wecho ose, but which do not admit foliations admitting any further re nements. For example, we nd manifolds which admit foliations without Reeb comp onents, but no taut foliations. We also nd 0 manifolds admitting C foliations with no compact leaves, but which do not admit 2 any C such foliations. These results p oint to the subtle nature b ehind b oth top ological and analytical assumptions when dealing with foliations. A principal motivation for this work came from a particularly interesting exam- ple; the manifold M obtained by 37/2 Dehn surgery on the 2,3,7 pretzel knot K . -
A TEXTBOOK of TOPOLOGY Lltld
SEIFERT AND THRELFALL: A TEXTBOOK OF TOPOLOGY lltld SEI FER T: 7'0PO 1.OG 1' 0 I.' 3- Dl M E N SI 0 N A I. FIRERED SPACES This is a volume in PURE AND APPLIED MATHEMATICS A Series of Monographs and Textbooks Editors: SAMUELEILENBERG AND HYMANBASS A list of recent titles in this series appears at the end of this volunie. SEIFERT AND THRELFALL: A TEXTBOOK OF TOPOLOGY H. SEIFERT and W. THRELFALL Translated by Michael A. Goldman und S E I FE R T: TOPOLOGY OF 3-DIMENSIONAL FIBERED SPACES H. SEIFERT Translated by Wolfgang Heil Edited by Joan S. Birman and Julian Eisner @ 1980 ACADEMIC PRESS A Subsidiary of Harcourr Brace Jovanovich, Publishers NEW YORK LONDON TORONTO SYDNEY SAN FRANCISCO COPYRIGHT@ 1980, BY ACADEMICPRESS, INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER. ACADEMIC PRESS, INC. 11 1 Fifth Avenue, New York. New York 10003 United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road, London NWI 7DX Mit Genehmigung des Verlager B. G. Teubner, Stuttgart, veranstaltete, akin autorisierte englische Ubersetzung, der deutschen Originalausgdbe. Library of Congress Cataloging in Publication Data Seifert, Herbert, 1897- Seifert and Threlfall: A textbook of topology. Seifert: Topology of 3-dimensional fibered spaces. (Pure and applied mathematics, a series of mono- graphs and textbooks ; ) Translation of Lehrbuch der Topologic. Bibliography: p. Includes index. 1. -
General Topology
General Topology Tom Leinster 2014{15 Contents A Topological spaces2 A1 Review of metric spaces.......................2 A2 The definition of topological space.................8 A3 Metrics versus topologies....................... 13 A4 Continuous maps........................... 17 A5 When are two spaces homeomorphic?................ 22 A6 Topological properties........................ 26 A7 Bases................................. 28 A8 Closure and interior......................... 31 A9 Subspaces (new spaces from old, 1)................. 35 A10 Products (new spaces from old, 2)................. 39 A11 Quotients (new spaces from old, 3)................. 43 A12 Review of ChapterA......................... 48 B Compactness 51 B1 The definition of compactness.................... 51 B2 Closed bounded intervals are compact............... 55 B3 Compactness and subspaces..................... 56 B4 Compactness and products..................... 58 B5 The compact subsets of Rn ..................... 59 B6 Compactness and quotients (and images)............. 61 B7 Compact metric spaces........................ 64 C Connectedness 68 C1 The definition of connectedness................... 68 C2 Connected subsets of the real line.................. 72 C3 Path-connectedness.......................... 76 C4 Connected-components and path-components........... 80 1 Chapter A Topological spaces A1 Review of metric spaces For the lecture of Thursday, 18 September 2014 Almost everything in this section should have been covered in Honours Analysis, with the possible exception of some of the examples. For that reason, this lecture is longer than usual. Definition A1.1 Let X be a set. A metric on X is a function d: X × X ! [0; 1) with the following three properties: • d(x; y) = 0 () x = y, for x; y 2 X; • d(x; y) + d(y; z) ≥ d(x; z) for all x; y; z 2 X (triangle inequality); • d(x; y) = d(y; x) for all x; y 2 X (symmetry). -
Spring 2000 Solutions to Midterm 2
Math 310 Topology, Spring 2000 Solutions to Midterm 2 Problem 1. A subspace A ⊂ X is called a retract of X if there is a map ρ: X → A such that ρ(a) = a for all a ∈ A. (Such a map is called a retraction.) Show that a retract of a Hausdorff space is closed. Show, further, that A is a retract of X if and only if the pair (X, A) has the following extension property: any map f : A → Y to any topological space Y admits an extension X → Y . Solution: (1) Let x∈ / A and a = ρ(x) ∈ A. Since X is Hausdorff, x and a have disjoint neighborhoods U and V , respectively. Then ρ−1(V ∩ A) ∩ U is a neighborhood of x disjoint from A. Hence, A is closed. (2) Let A be a retract of X and ρ: A → X a retraction. Then for any map f : A → Y the composition f ◦ρ: X → Y is an extension of f. Conversely, if (X, A) has the extension property, take Y = A and f = idA. Then any extension of f to X is a retraction. Problem 2. A compact Hausdorff space X is called an absolute retract if whenever X is embedded into a normal space Y the image of X is a retract of Y . Show that a compact Hausdorff space is an absolute retract if and only if there is an embedding X,→ [0, 1]J to some cube [0, 1]J whose image is a retract. (Hint: Assume the Tychonoff theorem and previous problem.) Solution: A compact Hausdorff space is normal, hence, completely regular, hence, it admits an embedding to some [0, 1]J . -
When Is the Natural Map a a Cofibration? Í22a
transactions of the american mathematical society Volume 273, Number 1, September 1982 WHEN IS THE NATURAL MAP A Í22A A COFIBRATION? BY L. GAUNCE LEWIS, JR. Abstract. It is shown that a map/: X — F(A, W) is a cofibration if its adjoint/: X A A -» W is a cofibration and X and A are locally equiconnected (LEC) based spaces with A compact and nontrivial. Thus, the suspension map r¡: X -» Ü1X is a cofibration if X is LEC. Also included is a new, simpler proof that C.W. complexes are LEC. Equivariant generalizations of these results are described. In answer to our title question, asked many years ago by John Moore, we show that 7j: X -> Í22A is a cofibration if A is locally equiconnected (LEC)—that is, the inclusion of the diagonal in A X X is a cofibration [2,3]. An equivariant extension of this result, applicable to actions by any compact Lie group and suspensions by an arbitrary finite-dimensional representation, is also given. Both of these results have important implications for stable homotopy theory where colimits over sequences of maps derived from r¡ appear unbiquitously (e.g., [1]). The force of our solution comes from the Dyer-Eilenberg adjunction theorem for LEC spaces [3] which implies that C.W. complexes are LEC. Via Corollary 2.4(b) below, this adjunction theorem also has some implications (exploited in [1]) for the geometry of the total spaces of the universal spherical fibrations of May [6]. We give a simpler, more conceptual proof of the Dyer-Eilenberg result which is equally applicable in the equivariant context and therefore gives force to our equivariant cofibration condition. -
Homology Groups of Homeomorphic Topological Spaces
An Introduction to Homology Prerna Nadathur August 16, 2007 Abstract This paper explores the basic ideas of simplicial structures that lead to simplicial homology theory, and introduces singular homology in order to demonstrate the equivalence of homology groups of homeomorphic topological spaces. It concludes with a proof of the equivalence of simplicial and singular homology groups. Contents 1 Simplices and Simplicial Complexes 1 2 Homology Groups 2 3 Singular Homology 8 4 Chain Complexes, Exact Sequences, and Relative Homology Groups 9 ∆ 5 The Equivalence of H n and Hn 13 1 Simplices and Simplicial Complexes Definition 1.1. The n-simplex, ∆n, is the simplest geometric figure determined by a collection of n n + 1 points in Euclidean space R . Geometrically, it can be thought of as the complete graph on (n + 1) vertices, which is solid in n dimensions. Figure 1: Some simplices Extrapolating from Figure 1, we see that the 3-simplex is a tetrahedron. Note: The n-simplex is topologically equivalent to Dn, the n-ball. Definition 1.2. An n-face of a simplex is a subset of the set of vertices of the simplex with order n + 1. The faces of an n-simplex with dimension less than n are called its proper faces. 1 Two simplices are said to be properly situated if their intersection is either empty or a face of both simplices (i.e., a simplex itself). By \gluing" (identifying) simplices along entire faces, we get what are known as simplicial complexes. More formally: Definition 1.3. A simplicial complex K is a finite set of simplices satisfying the following condi- tions: 1 For all simplices A 2 K with α a face of A, we have α 2 K. -
Algebraic Obstructions and a Complete Solution of a Rational Retraction Problem
PROCEEDINGS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 130, Number 12, Pages 3525{3535 S 0002-9939(02)06617-0 Article electronically published on May 15, 2002 ALGEBRAIC OBSTRUCTIONS AND A COMPLETE SOLUTION OF A RATIONAL RETRACTION PROBLEM RICCARDO GHILONI (Communicated by Paul Goerss) Abstract. For each compact smooth manifold W containing at least two points we prove the existence of a compact nonsingular algebraic set Z and a smooth map g : Z W such that, for every rational diffeomorphism −→ r : Z Z and for every diffeomorphism s : W W where Z and 0 −→ 0 −→ 0 W are compact nonsingular algebraic sets, we may fix a neighborhood of 0 U s 1 g r in C (Z ;W ) which does not contain any regular rational map. − ◦ ◦ 1 0 0 Furthermore s 1 g r is not homotopic to any regular rational map. Bearing − ◦ ◦ inmindthecaseinwhichW is a compact nonsingular algebraic set with totally algebraic homology, the previous result establishes a clear distinction between the property of a smooth map f to represent an algebraic unoriented bordism class and the property of f to be homotopic to a regular rational map. Furthermore we have: every compact Nash submanifold of Rn containing at least two points has not any tubular neighborhood with rational retraction. 1. Introduction This paper deals with algebraic obstructions. First, it is necessary to recall part of the classical problem of making smooth objects algebraic. Let M be a smooth manifold. A nonsingular real algebraic set V diffeomorphic to M is called an algebraic model of M. In [28] Tognoli proved that any compact smooth manifold has an algebraic model. -
The Orientation-Preserving Diffeomorphism Group of S^ 2
THE ORIENTATION-PRESERVING DIFFEOMORPHISM GROUP OF S2 DEFORMS TO SO(3) SMOOTHLY JIAYONG LI AND JORDAN ALAN WATTS Abstract. Smale proved that the orientation-preserving diffeomorphism group of S2 has a continuous strong deformation retraction to SO(3). In this paper, we construct such a strong deformation retraction which is diffeologically smooth. 1. Introduction In Smale’s 1959 paper “Diffeomorphisms of the 2-Sphere” ([8]), he shows that there is a continuous strong deformation retraction from the orientation- preserving C∞ diffeomorphism group of S2 to the rotation group SO(3). The topology of the former is the Ck topology. In this paper, we construct such a strong deformation retraction which is diffeologically smooth. We follow the general idea of [8], but to achieve smoothness, some of the steps we use are completely different from those of [8]. The most notable differences are explained in Remark 2.3, 2.5, and Remark 3.4. We note that there is a different proof of Smale’s result in [2], but the homotopy is not shown to be smooth. We start by defining the notion of diffeological smoothness in three special cases which are directly applicable to this paper. Note that diffeology can be defined in a much more general context and we refer the readers to [4]. Definition 1.1. Let U be an arbitrary open set in a Euclidean space of arbitrary dimension. arXiv:0912.2877v2 [math.DG] 1 Apr 2010 • Suppose Λ is a manifold with corners. A map P : U → Λ is a plot if P is C∞. • Suppose X and Y are manifolds with corners, and Λ ⊂ C∞(X,Y ). -
Fibrations I
Fibrations I Tyrone Cutler May 23, 2020 Contents 1 Fibrations 1 2 The Mapping Path Space 5 3 Mapping Spaces and Fibrations 9 4 Exercises 11 1 Fibrations In the exercises we used the extension problem to motivate the study of cofibrations. The idea was to allow for homotopy-theoretic methods to be introduced to an otherwise very rigid problem. The dual notion is the lifting problem. Here p : E ! B is a fixed map and we would like to known when a given map f : X ! B lifts through p to a map into E E (1.1) |> | p | | f X / B: Asking for the lift to make the diagram to commute strictly is neither useful nor necessary from our point of view. Rather it is more natural for us to ask that the lift exist up to homotopy. In this lecture we work in the unpointed category and obtain the correct conditions on the map p by formally dualising the conditions for a map to be a cofibration. Definition 1 A map p : E ! B is said to have the homotopy lifting property (HLP) with respect to a space X if for each pair of a map f : X ! E, and a homotopy H : X×I ! B starting at H0 = pf, there exists a homotopy He : X × I ! E such that , 1) He0 = f 2) pHe = H. The map p is said to be a (Hurewicz) fibration if it has the homotopy lifting property with respect to all spaces. 1 Since a diagram is often easier to digest, here is the definition exactly as stated above f X / E x; He x in0 x p (1.2) x x H X × I / B and also in its equivalent adjoint formulation X B H B He B B p∗ EI / BI (1.3) f e0 e0 # p E / B: The assertion that p is a fibration is the statement that the square in the second diagram is a weak pullback. -
Math 396. Interior, Closure, and Boundary We Wish to Develop Some Basic Geometric Concepts in Metric Spaces Which Make Precise C
Math 396. Interior, closure, and boundary We wish to develop some basic geometric concepts in metric spaces which make precise certain intuitive ideas centered on the themes of \interior" and \boundary" of a subset of a metric space. One warning must be given. Although there are a number of results proven in this handout, none of it is particularly deep. If you carefully study the proofs (which you should!), then you'll see that none of this requires going much beyond the basic definitions. We will certainly encounter some serious ideas and non-trivial proofs in due course, but at this point the central aim is to acquire some linguistic ability when discussing some basic geometric ideas in a metric space. Thus, the main goal is to familiarize ourselves with some very convenient geometric terminology in terms of which we can discuss more sophisticated ideas later on. 1. Interior and closure Let X be a metric space and A X a subset. We define the interior of A to be the set ⊆ int(A) = a A some Br (a) A; ra > 0 f 2 j a ⊆ g consisting of points for which A is a \neighborhood". We define the closure of A to be the set A = x X x = lim an; with an A for all n n f 2 j !1 2 g consisting of limits of sequences in A. In words, the interior consists of points in A for which all nearby points of X are also in A, whereas the closure allows for \points on the edge of A".