DOCSLIB.ORG

September 8 Homeomorphisms. As I Mentioned in the First Lecture, The

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
September 8 Homeomorphisms. As I Mentioned in the First Lecture, The 1 Lecture 4: September 8 Homeomorphisms. As I mentioned in the first lecture, the purpose of topology is to look at qualitative properties of geometric objects that do not depend on the exact shape of an object, but more on how the object is put together. We formalized this idea by definiting topological spaces; but what does it mean to say that two di↵erent topological spaces (such as a circle and a square) are really “the same”? Definition 4.1. Let f : X Y be a bijective function between topological spaces. ! 1 If both f and the inverse function f − : Y X are continuous, then f is called a homeomorphism, and X and Y are said to! be homeomorphic. Intuitively, think of X as being made from some elastic material (like a balloon), and think of stretching, bending, or otherwise changing the shape of X without tearing the material. Any Y that you get in this way will be homeomorphic to the original X. Note that the actual definition is both more precise and more general, since we are allowing arbitrary functions. Suppose that f : X Y is a homeomorphism. For each open set U X,we ! 1 ✓ are assuming that its inverse image under f − : Y X is open in X; but because f is bijective, this is the same as the image of U! under f. In other words, a homeomorphism is a bijective function f : X Y such that f(U) is open if and only if U is open. We therefore get a bijective! correspondence not only between the points of X and Y , but also between the open sets in both topologies. So any question about the topology of X or Y will have the same answer on both sides; we may therefore think of X and Y as being essentially the same topological space. Example 4.2. The real numbers R are homemorphic to the open interval (0, 1). One possible choice of homeomorphism is the function ex f : R (0, 1),f(x)= ! ex +1 1 Both f and the inverse function f − (y) = log(y) log(1 y) are continuous. − − Example 4.3. Consider the function 1 f :[0, 1) S ,f(t) = (cos t, sin t) ! that takes the interval (with the subspace topology from R)totheunitcircle (with the subspace topology from R2). It is bijective and continuous, but not a homeomorphism: [0, 1/2) is open in [0, 1), but its image is not open in S1. Example 4.4. Let us classify the letters of the English alphabet ABCDEFGHIJKLMNOPQRSTUVWXYZ up to homeomorphism. Here we think of each letter as being made from line segments in R2; the topology is the subspace topology. By inspection, there are eight homeomorphism classes, depending on the number of loops and line segments in each letter: BARPQDOCGIJLMNSUVWZEFTYHKX For example, W can be bent to make I, and so the two are homeomorphic. On the other hand, there is no homeomorphism between T and I: if we remove the crossing point, we are left with three intervals in the case of T, but removing one point from I produces at most two intervals. (Think about how one can say this in terms of the topology on each letter.) 2 Topological manifolds. In the remainder of today’s class, I want to introduce three additional examples of topological spaces. The first one is topological mani- folds. A manifold is a space X that “locally” looks like Euclidean space: if you sit at any point of X, and only look at points nearby, you may think that you are in Rn. Here is the precise definition. Definition 4.5. An n-dimensional topological manifold is a Hausdor↵topological space X with the following property: every point x X has a neighborhood that 2 is homeomorphic to an open subset in Rn. In geometry, people look at other classes of manifolds that are obtained by working with a smaller class of functions. For example, if a function and its inverse function are both di↵erentiable, it is called a di↵eomorphism; di↵erentiable man- ifolds are defined by replacing “homeomorphic” by “di↵eomorphic” in the above definition. In algebraic geometry, there is a similar definition with polynomials. At this point, somebody asked why we need the Hausdor↵condition; the answer is that we do not want to allow something like taking two copies of R and gluing them together along R 0 . (More about this example later on, when we discuss \{ } quotient spaces.) Later in the semester, we will show that an open subset in Rn can never be homeomorphic to an open subset in Rm for m = n; this means that the dimension of a manifold really is a well-defined notion. 6 Example 4.6. The square and the circle are both one-dimensional manifolds; a homeomorphism between them is given by drawing the square inside the circle and projecting one to the other from their common center. Example 4.7. The n-sphere n n+1 2 2 2 S = (x0,x1,...,xn) R x + x + + x =1 , 2 0 1 ··· n with the subspace topology coming from Rn+1 , is an n-dimensional manifold. Intu- itively, this is clear; let me prove it for n = 2 by using stereographic projection. The plane z = 1 is tangent to the sphere at the south pole; given any point (x, y, z) not equal to− the north pole (0, 0, 1), we can see where the line connecting (0, 0, 1) and (x, y, z) intersects the plane z = 1. In this way, we get a bijection − 2 2 f : S (0, 0, 1) R . \{ }! It is easy to work out the formulas to see that f and its inverse are continuous. The points on the line are parametrized by (0, 0, 1) + t(x, y, z 1), with t R;the intersection point with the plane has − 2 2 1+t(z 1) = 1 or t = , − − 1 z − which means that 2x 2y f(x, y, z)= , . 1 z 1 z −1 − One can show in a similar manner that f − is continuous. Since we can also do stereographic projection from the south pole, every point of S2 has a neighborhood 2 that is homeomorphic to R . Å ã Example 4.8. The inverse function theorem from analysis gives us one way to define manifolds. Suppose that f : R2 R is a continuously di↵erentiable function. The ! inverse function theorem says that if f(x0,y0) = 0, and if the partial derivative 3 @f/@y does not vanish at the point (x0,y0), then all nearby solutions of the equation f(x, y) = 0 are of the form y = '(x) for a continously di↵erentiable function ':(x0 ", x0 + ") R with '(x0)=y0. This function ' gives us a homeomorphism between− a small! neighborhood of the 1 1 point (x0,y0)inthesetf − (0) and an open interval in R. This shows that f − (0) is a one-dimensional manifold, provided that at least one of the two partial derivatives 1 @f/@x or @f/@y is nonzero at every point of f − (0). Example 4.9. If M1 and M2 are manifolds of dimension n1 and n2,respectively, then their product M1 M2 (with the product topology) is a manifold of dimension ⇥ 1 1 n1 + n2. The proof is left as an exercise. For instance, the product S S is a two-dimensional manifold called the torus. ⇥ An important general problem is to classify manifolds (or more general topolog- ical spaces) up to homeomorphism. In general, this is only possible if we impose sufficiently many other conditions (such as connectedness or compactness) to limit the class of topological spaces we are looking at. We will come back to this problem later in the semester. Quotient spaces and the quotient topology. In geometry, it is common to describe spaces by “cut-and-paste” constructions like the following. Example 4.10. If we start from the unit square and paste opposite edges (with the same orientation), we get the torus. If we start from the closed unit disk in R2 and collapse the entire boundary into a point, we obtain S2. To make a M¨obius band, we take a strip of paper, twist one end by 180◦, and then glue the two ends together. We can make a torus with two holes by taking two copies of the torus, removing a small disk from each, and then pasting them together along the two boundary circles. In each of these cases, the result should again be a topological space. To formalize this type of construction, we start with a topological space X and an equivalence relation on it; intuitively, tells us which points of X should be glued together. (Recall that⇠ an equivalence⇠ relation is the same thing as a partition of X into disjoint subsets, namely the equivalence classes; two points are equivalent if and only if they are in the same equivalence class.) What we want to do is to build a new topological space in which each equivalence class becomes just one point. To do this, we let X/ be the set of equivalence classes; there is an obvious function ⇠ p: X X/ , ! ⇠ which takes a point x X to the equivalence class containing x. Now there is a natural way to make X/2 into a topological space. ⇠ Proposition 4.11. The collection of sets 1 T = U X/ p− (U) is open in X ✓ ⇠ defines a topology on X/ ,calledthequotient topology. ⇠ Proof. We have to check that the three conditions in the definition of topology are 1 1 satisfied.
Load more

Recommended publications
  • ON Hg-HOMEOMORPHISMS in TOPOLOGICAL SPACES
    Proyecciones Vol. 28, No 1, pp. 1—19, May 2009. Universidad Cat´olica del Norte Antofagasta - Chile ON g-HOMEOMORPHISMS IN TOPOLOGICAL SPACES e M. CALDAS UNIVERSIDADE FEDERAL FLUMINENSE, BRASIL S. JAFARI COLLEGE OF VESTSJAELLAND SOUTH, DENMARK N. RAJESH PONNAIYAH RAMAJAYAM COLLEGE, INDIA and M. L. THIVAGAR ARUL ANANDHAR COLLEGE Received : August 2006. Accepted : November 2008 Abstract In this paper, we first introduce a new class of closed map called g-closed map. Moreover, we introduce a new class of homeomor- phism called g-homeomorphism, which are weaker than homeomor- phism.e We prove that gc-homeomorphism and g-homeomorphism are independent.e We also introduce g*-homeomorphisms and prove that the set of all g*-homeomorphisms forms a groupe under the operation of composition of maps. e e 2000 Math. Subject Classification : 54A05, 54C08. Keywords and phrases : g-closed set, g-open set, g-continuous function, g-irresolute map. e e e e 2 M. Caldas, S. Jafari, N. Rajesh and M. L. Thivagar 1. Introduction The notion homeomorphism plays a very important role in topology. By definition, a homeomorphism between two topological spaces X and Y is a 1 bijective map f : X Y when both f and f − are continuous. It is well known that as J¨anich→ [[5], p.13] says correctly: homeomorphisms play the same role in topology that linear isomorphisms play in linear algebra, or that biholomorphic maps play in function theory, or group isomorphisms in group theory, or isometries in Riemannian geometry. In the course of generalizations of the notion of homeomorphism, Maki et al.
    [Show full text]
  • 07. Homeomorphisms and Embeddings
    07. Homeomorphisms and embeddings Homeomorphisms are the isomorphisms in the category of topological spaces and continuous functions. Definition. 1) A function f :(X; τ) ! (Y; σ) is called a homeomorphism between (X; τ) and (Y; σ) if f is bijective, continuous and the inverse function f −1 :(Y; σ) ! (X; τ) is continuous. 2) (X; τ) is called homeomorphic to (Y; σ) if there is a homeomorphism f : X ! Y . Remarks. 1) Being homeomorphic is apparently an equivalence relation on the class of all topological spaces. 2) It is a fundamental task in General Topology to decide whether two spaces are homeomorphic or not. 3) Homeomorphic spaces (X; τ) and (Y; σ) cannot be distinguished with respect to their topological structure because a homeomorphism f : X ! Y not only is a bijection between the elements of X and Y but also yields a bijection between the topologies via O 7! f(O). Hence a property whose definition relies only on set theoretic notions and the concept of an open set holds if and only if it holds in each homeomorphic space. Such properties are called topological properties. For example, ”first countable" is a topological property, whereas the notion of a bounded subset in R is not a topological property. R ! − t Example. The function f : ( 1; 1) with f(t) = 1+jtj is a −1 x homeomorphism, the inverse function is f (x) = 1−|xj . 1 − ! b−a a+b The function g :( 1; 1) (a; b) with g(x) = 2 x + 2 is a homeo- morphism. Therefore all open intervals in R are homeomorphic to each other and homeomorphic to R .
    [Show full text]
  • Topologically Rigid (Surgery/Geodesic Flow/Homeomorphism Group/Marked Leaves/Foliated Control) F
    Proc. Natl. Acad. Sci. USA Vol. 86, pp. 3461-3463, May 1989 Mathematics Compact negatively curved manifolds (of dim # 3, 4) are topologically rigid (surgery/geodesic flow/homeomorphism group/marked leaves/foliated control) F. T. FARRELLt AND L. E. JONESt tDepartment of Mathematics, Columbia University, New York, NY 10027; and tDepartment of Mathematics, State University of New York, Stony Brook, NY 11794 Communicated by William Browder, January 27, 1989 (receivedfor review October 20, 1988) ABSTRACT Let M be a complete (connected) Riemannian COROLLARY 1.3. If M and N are both negatively curved, manifold having finite volume and whose sectional curvatures compact (connected) Riemannian manifolds with isomorphic lie in the interval [cl, c2d with -x < cl c2 < 0. Then any fundamental groups, then M and N are homeomorphic proper homotopy equivalence h:N -* M from a topological provided dim M 4 3 and 4. manifold N is properly homotopic to a homeomorphism, Gromov had previously shown (under the hypotheses of provided the dimension of M is >5. In particular, if M and N Corollary 1.3) that the total spaces of the tangent sphere are both compact (connected) negatively curved Riemannian bundles ofM and N are homeomorphic (even when dim M = manifolds with isomorphic fundamental groups, then M and N 3 or 4) by a homeomorphism preserving the orbits of the are homeomorphic provided dim M # 3 and 4. {If both are geodesic flows, and Cheeger had (even earlier) shown that locally symmetric, this is a consequence of Mostow's rigidity the total spaces of the two-frame bundles of M and N are theorem [Mostow, G.
    [Show full text]
  • (X,TX) and (Y,TY ) Is A
    Homeomorphisms. A homeomorphism between two topological spaces (X; X ) and (Y; Y ) is a one 1 T T - one correspondence such that f and f − are both continuous. Consequently, for every U X there is V Y such that V = F (U) and 1 2 T 2 T U = f − (V ). Furthermore, since f(U1 U2) = f(U1) f(U2) \ \ and f(U1 U2) = f(U1) f(U2) [ [ this equivalence extends to the structures of the spaces. Example Any open interval in R (with the inherited topology) is homeomorphic to R. One possible function from R to ( 1; 1) is f(x) = tanh x, and by suitable scaling this provides a homeomorphism onto− any finite open interval (a; b). b + a b a f(x) = + − tanh x 2 2 For semi-infinite intervals we can use with f(x) = a + ex from R to (a; ) and x 1 f(x) = b e− from R to ( ; b). − −∞ Since homeomorphism is an equivalence relation, this shows that all open inter- vals in R are homeomorphic. nb The functions chosen above are not unique. On the other hand, no closed interval [a; b] is homeomorphic to R, since such an interval is compact in R, and hence f([a; b]) is compact for any continuous function f. Example 2 The function f(z) = z2 induces a homeomorphism between the complex plane C and a two sheeted Riemann surface, which we can construct piecewise using f. Consider first the half-plane x > 0 C. The function f maps this half plane⊂ onto a complex plane missing the negative real axis.
    [Show full text]
  • 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.
    [Show full text]
  • 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).
    [Show full text]
  • On Generalized Homeomorphisms in Topological Spaces
    Divulgaciones Matem´aticasVol. 9 No. 1(2001), pp. 55–63 More on Generalized Homeomorphisms in Topological Spaces M´asSobre Homeomorfismos Generalizados en Espacios Topol´ogicos Miguel Caldas ([email protected]) Departamento de Matem´aticaAplicada Universidade Federal Fluminense-IMUFF Rua M´arioSantos Braga s/n0; CEP:24020-140, Niteroi-RJ. Brasil. Abstract The aim of this paper is to continue the study of generalized home- omorphisms. For this we define three new classes of maps, namely c generalized Λs-open, generalized Λs-homeomorphisms and generalized I Λs-homeomorphisms, by using g:Λs-sets, which are generalizations of semi-open maps and generalizations of homeomorphisms. Key words and phrases: Topological spaces, semi-closed sets, semi- open sets, semi-homeomorphisms, semi-closed maps, irresolute maps. Resumen El objetivo de este trabajo es continuar el estudio de los homeomor- fismos generalizados. Para esto definimos tres nuevas clases de aplica- c ciones, denominadas Λs-abiertas generalizadas, Λs-homeomofismos ge- I neralizados y Λs-homeomorfismos generalizados, haciendo uso de los conjuntos g:Λs, los cuales son generalizaciones de las funciones semi- abiertas y generalizaciones de homeomorfismos. Palabras y frases clave: Espacios topol´ogicos,conjuntos semi-cerra- dos, conjuntos semi-abiertos, semi-homeomorfismos, aplicaciones semi- cerradas, aplicaciones irresolutas. Recibido 2000/10/23. Revisado 2001/03/20. Aceptado 2001/03/29. MSC (2000): Primary 54C10, 54D10. 56 Miguel Caldas 1 Introduction Recently in 1998, as an analogy of Maki [10],
    [Show full text]
  • Lecture 6 HOMOTOPY the Notions of Homotopy and Homotopy Equivalence Are Quite Fundamental in Topology. Homotopy Equivalence of T
    Lecture 6 HOMOTOPY The notions of homotopy and homotopy equivalence are quite fundamental in topology. Homotopy equivalence of topological spaces is a weaker equivalence relation than homeomorphism, and homotopy theory studies topological spaces up to this relation (and maps up to homotopy). This theory constitutes the main body of algebraic topology, but we only consider a few of its basic notions here. One of these notions is the Euler characteristic, which is also a homotopy invariant. 6.1. Homotopic Maps Two maps f, g : X ! Y are called homotopic (notation f ' g) if they can be joined by a homotopy, i.e., by a map F : X × [0; 1] ! Y such that F (x; 0) ≡ f(x) and F (x; 1) ≡ g(x) (here ≡ means for all x 2 X). If we change the notation from F (x; t) to Ft(x), we can restate the previous definition by saying that there exists a family fFt(x)g of maps, parametrized by t 2 [0; 1], continiously changing from f ≡ F0 to g ≡ F1. It is easy to prove that f ' f for any f : X ! Y (reflexivity); f ' g =) f ' g for all f, g : X ! Y (symmetry); f ' g and g ' h =) f ' h for all f, g, h: X ! Y (transitivity). For example, to prove transitivity, we obtain a homotopy joining f and h by setting ( F (x; 2t) for 0 ≤ t ≤ 1=2; F (x; t) = 1 F2(x; 2t − 1) for 1=2 ≤ t ≤ 1, where F1, F2 are homotopies joining f and g, g and h, respectively.
    [Show full text]
  • Homeomorphisms Between Topological Manifolds and Analytic Manifolds Author(S): Stewart S
    Annals of Mathematics Homeomorphisms Between Topological Manifolds and Analytic Manifolds Author(s): Stewart S. Cairns Source: The Annals of Mathematics, Second Series, Vol. 41, No. 4 (Oct., 1940), pp. 796-808 Published by: Annals of Mathematics Stable URL: http://www.jstor.org/stable/1968860 Accessed: 15/12/2010 04:49 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=annals. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Annals of Mathematics is collaborating with JSTOR to digitize, preserve and extend access to The Annals of Mathematics. http://www.jstor.org ANNA oFMv ATEMlATQCS Vol. 41, No.
    [Show full text]
  • Topology - Wikipedia, the Free Encyclopedia Page 1 of 7
    Topology - Wikipedia, the free encyclopedia Page 1 of 7 Topology From Wikipedia, the free encyclopedia Topology (from the Greek τόπος , “place”, and λόγος , “study”) is a major area of mathematics concerned with properties that are preserved under continuous deformations of objects, such as deformations that involve stretching, but no tearing or gluing. It emerged through the development of concepts from geometry and set theory, such as space, dimension, and transformation. Ideas that are now classified as topological were expressed as early as 1736. Toward the end of the 19th century, a distinct A Möbius strip, an object with only one discipline developed, which was referred to in Latin as the surface and one edge. Such shapes are an geometria situs (“geometry of place”) or analysis situs object of study in topology. (Greek-Latin for “picking apart of place”). This later acquired the modern name of topology. By the middle of the 20 th century, topology had become an important area of study within mathematics. The word topology is used both for the mathematical discipline and for a family of sets with certain properties that are used to define a topological space, a basic object of topology. Of particular importance are homeomorphisms , which can be defined as continuous functions with a continuous inverse. For instance, the function y = x3 is a homeomorphism of the real line. Topology includes many subfields. The most basic and traditional division within topology is point-set topology , which establishes the foundational aspects of topology and investigates concepts inherent to topological spaces (basic examples include compactness and connectedness); algebraic topology , which generally tries to measure degrees of connectivity using algebraic constructs such as homotopy groups and homology; and geometric topology , which primarily studies manifolds and their embeddings (placements) in other manifolds.
    [Show full text]
  • Homotopy Equivalence and Homeomorphism of 3-Manifolds
    oo‘s9383192 ss.00 + 0.m Q IWZ Pcrgamoo Rcu Ltd HOMOTOPY EQUIVALENCE AND HOMEOMORPHISM OF 3-MANIFOLDS JOEL HAsst and PETER SCOTT$ (Receired 10 Septrmber 1990; in revised form 9 August 1991) $1. INTRODUCTION AND PRELIMINARY RESULTS IN THIS paper we extend the class of 3-manifolds which are determined up to homeomor- phism by their fundamental groups to the class of closed orientable irreducible 3-manifolds containing a singular surface satisfying two properties, the l-line-intersection property and the 4-plane property. A basic problem in the classification of 3-dimensional manifolds is to decide to what extent the homotopy type of a closed manifold determines the manifold up to homeomor- phism. In the case of 3-manifolds with finite fundamental groups, it is known that there are homotopy equivalent manifolds which are not homeomorphic, but there are no known examples of closed orientable irreducible 3-manifolds with isomorphic infinite fundamental groups which are not homcomorphic. Waldhausen [30] and Heil Cl23 proved that if M and M’ are Haken 3-manifolds which have isomorphic fundamental groups then they are homcomorphic. If M and M’ arc hyperbolic then the Mostow rigidity theorem implies the same result [19]. but if only M is assumed to be hyperbolic then it is unknown. Boehme [3] extended Waldhausen’s theorem to certain non-Haken Seifert fiber spaces. Scott [27] showed that a closed orientable irreducible 3-manifold which is homotopy equivalent to a Seifcrt fiber space with infinite fundamental group is homeomorphic to that Seifert fiber space. Many of these Scifert fiber spaces are non-Haken.
    [Show full text]
  • Automatic Continuity for Homeomorphism Groups and Applications
    Automatic continuity for homeomorphism groups and applications Kathryn Mann Appendix with Fr´ed´ericLe Roux Abstract Let M be a compact manifold, possibly with boundary. We show that the group of homeomorphisms of M has the automatic continuity property: any homomorphism from Homeo(M) to any separable group is necessarily continuous. This answers a question of C. Rosendal. If N ⊂ M is a submanifold, the group of homeomorphisms of M that preserve N also has this property. Various applications of automatic continuity are discussed, including applications to the topology and structure of groups of germs of homeomorphisms. In an appendix with Fr´ed´ericLe Roux we also show, using related techniques, that the group of germs at a point of homeomorphisms of Rn is strongly uniformly simple. 1 Introduction Definition 1.1. A topological group G has the automatic continuity property if every homomorphism from G to any separable group H is necessarily continuous. One should think of automatic continuity as a very strong form of rigidity. Many familiar topological groups fail to have the property, for example • Automorphisms of R as a vector space over Q (other than homotheties) are discon- tinuous homomorphisms R ! R. • Applying a wild automorphism of C to all matrix entries gives a discontinuous homomorphism GL(n; C) ! GL(n; C). • More generally, for any field F of cardinality at most continuum, Kallman [11] gives injective homomorphisms from GL(n; F ) to S1, the group of permutations of an infinite countable set. As S1 admits a separable, totally disconnected topology, GL(n; F ) fails to have automatic continuity as soon as F is not totally disconnected.
    [Show full text]