Math 396. Quotients by Group Actions Many Important Manifolds Are
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Math 5853 Homework Solutions 36. a Map F : X → Y Is Called an Open
Math 5853 homework solutions 36. A map f : X → Y is called an open map if it takes open sets to open sets, and is called a closed map if it takes closed sets to closed sets. For example, a continuous bijection is a homeomorphism if and only if it is a closed map and an open map. 1. Give examples of continuous maps from R to R that are open but not closed, closed but not open, and neither open nor closed. open but not closed: f(x) = ex is a homeomorphism onto its image (0, ∞) (with the logarithm function as its inverse). If U is open, then f(U) is open in (0, ∞), and since (0, ∞) is open in R, f(U) is open in R. Therefore f is open. However, f(R) = (0, ∞) is not closed, so f is not closed. closed but not open: Constant functions are one example. We will give an example that is also surjective. Let f(x) = 0 for −1 ≤ x ≤ 1, f(x) = x − 1 for x ≥ 1, and f(x) = x+1 for x ≤ −1 (f is continuous by “gluing together on a finite collection of closed sets”). f is not open, since (−1, 1) is open but f((−1, 1)) = {0} is not open. To show that f is closed, let C be any closed subset of R. For n ∈ Z, −1 −1 define In = [n, n + 1]. Now, f (In) = [n + 1, n + 2] if n ≥ 1, f (I0) = [−1, 2], −1 −1 f (I−1) = [−2, 1], and f (In) = [n − 1, n] if n ≤ −2. -
Math 5111 (Algebra 1) Lecture #14 of 24 ∼ October 19Th, 2020
Math 5111 (Algebra 1) Lecture #14 of 24 ∼ October 19th, 2020 Group Isomorphism Theorems + Group Actions The Isomorphism Theorems for Groups Group Actions Polynomial Invariants and An This material represents x3.2.3-3.3.2 from the course notes. Quotients and Homomorphisms, I Like with rings, we also have various natural connections between normal subgroups and group homomorphisms. To begin, observe that if ' : G ! H is a group homomorphism, then ker ' is a normal subgroup of G. In fact, I proved this fact earlier when I introduced the kernel, but let me remark again: if g 2 ker ', then for any a 2 G, then '(aga−1) = '(a)'(g)'(a−1) = '(a)'(a−1) = e. Thus, aga−1 2 ker ' as well, and so by our equivalent properties of normality, this means ker ' is a normal subgroup. Thus, we can use homomorphisms to construct new normal subgroups. Quotients and Homomorphisms, II Equally importantly, we can also do the reverse: we can use normal subgroups to construct homomorphisms. The key observation in this direction is that the map ' : G ! G=N associating a group element to its residue class / left coset (i.e., with '(a) = a) is a ring homomorphism. Indeed, the homomorphism property is precisely what we arranged for the left cosets of N to satisfy: '(a · b) = a · b = a · b = '(a) · '(b). Furthermore, the kernel of this map ' is, by definition, the set of elements in G with '(g) = e, which is to say, the set of elements g 2 N. Thus, kernels of homomorphisms and normal subgroups are precisely the same things. -
The Fundamental Groupoid of the Quotient of a Hausdorff
The fundamental groupoid of the quotient of a Hausdorff space by a discontinuous action of a discrete group is the orbit groupoid of the induced action Ronald Brown,∗ Philip J. Higgins,† Mathematics Division, Department of Mathematical Sciences, School of Informatics, Science Laboratories, University of Wales, Bangor South Rd., Gwynedd LL57 1UT, U.K. Durham, DH1 3LE, U.K. October 23, 2018 University of Wales, Bangor, Maths Preprint 02.25 Abstract The main result is that the fundamental groupoidof the orbit space of a discontinuousaction of a discrete groupon a Hausdorffspace which admits a universal coveris the orbit groupoid of the fundamental groupoid of the space. We also describe work of Higgins and of Taylor which makes this result usable for calculations. As an example, we compute the fundamental group of the symmetric square of a space. The main result, which is related to work of Armstrong, is due to Brown and Higgins in 1985 and was published in sections 9 and 10 of Chapter 9 of the first author’s book on Topology [3]. This is a somewhat edited, and in one point (on normal closures) corrected, version of those sections. Since the book is out of print, and the result seems not well known, we now advertise it here. It is hoped that this account will also allow wider views of these results, for example in topos theory and descent theory. Because of its provenance, this should be read as a graduate text rather than an article. The Exercises should be regarded as further propositions for which we leave the proofs to the reader. -
GROUP ACTIONS 1. Introduction the Groups Sn, An, and (For N ≥ 3)
GROUP ACTIONS KEITH CONRAD 1. Introduction The groups Sn, An, and (for n ≥ 3) Dn behave, by their definitions, as permutations on certain sets. The groups Sn and An both permute the set f1; 2; : : : ; ng and Dn can be considered as a group of permutations of a regular n-gon, or even just of its n vertices, since rigid motions of the vertices determine where the rest of the n-gon goes. If we label the vertices of the n-gon in a definite manner by the numbers from 1 to n then we can view Dn as a subgroup of Sn. For instance, the labeling of the square below lets us regard the 90 degree counterclockwise rotation r in D4 as (1234) and the reflection s across the horizontal line bisecting the square as (24). The rest of the elements of D4, as permutations of the vertices, are in the table below the square. 2 3 1 4 1 r r2 r3 s rs r2s r3s (1) (1234) (13)(24) (1432) (24) (12)(34) (13) (14)(23) If we label the vertices in a different way (e.g., swap the labels 1 and 2), we turn the elements of D4 into a different subgroup of S4. More abstractly, if we are given a set X (not necessarily the set of vertices of a square), then the set Sym(X) of all permutations of X is a group under composition, and the subgroup Alt(X) of even permutations of X is a group under composition. If we list the elements of X in a definite order, say as X = fx1; : : : ; xng, then we can think about Sym(X) as Sn and Alt(X) as An, but a listing in a different order leads to different identifications 1 of Sym(X) with Sn and Alt(X) with An. -
Lie Group and Geometry on the Lie Group SL2(R)
INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR Lie group and Geometry on the Lie Group SL2(R) PROJECT REPORT – SEMESTER IV MOUSUMI MALICK 2-YEARS MSc(2011-2012) Guided by –Prof.DEBAPRIYA BISWAS Lie group and Geometry on the Lie Group SL2(R) CERTIFICATE This is to certify that the project entitled “Lie group and Geometry on the Lie group SL2(R)” being submitted by Mousumi Malick Roll no.-10MA40017, Department of Mathematics is a survey of some beautiful results in Lie groups and its geometry and this has been carried out under my supervision. Dr. Debapriya Biswas Department of Mathematics Date- Indian Institute of Technology Khargpur 1 Lie group and Geometry on the Lie Group SL2(R) ACKNOWLEDGEMENT I wish to express my gratitude to Dr. Debapriya Biswas for her help and guidance in preparing this project. Thanks are also due to the other professor of this department for their constant encouragement. Date- place-IIT Kharagpur Mousumi Malick 2 Lie group and Geometry on the Lie Group SL2(R) CONTENTS 1.Introduction ................................................................................................... 4 2.Definition of general linear group: ............................................................... 5 3.Definition of a general Lie group:................................................................... 5 4.Definition of group action: ............................................................................. 5 5. Definition of orbit under a group action: ...................................................... 5 6.1.The general linear -
Definition 1.1. a Group Is a Quadruple (G, E, ⋆, Ι)
GROUPS AND GROUP ACTIONS. 1. GROUPS We begin by giving a definition of a group: Definition 1.1. A group is a quadruple (G, e, ?, ι) consisting of a set G, an element e G, a binary operation ?: G G G and a map ι: G G such that ∈ × → → (1) The operation ? is associative: (g ? h) ? k = g ? (h ? k), (2) e ? g = g ? e = g, for all g G. ∈ (3) For every g G we have g ? ι(g) = ι(g) ? g = e. ∈ It is standard to suppress the operation ? and write gh or at most g.h for g ? h. The element e is known as the identity element. For clarity, we may also write eG instead of e to emphasize which group we are considering, but may also write 1 for e where this is more conventional (for the group such as C∗ for example). Finally, 1 ι(g) is usually written as g− . Remark 1.2. Let us note a couple of things about the above definition. Firstly clo- sure is not an axiom for a group whatever anyone has ever told you1. (The reason people get confused about this is related to the notion of subgroups – see Example 1.8 later in this section.) The axioms used here are not “minimal”: the exercises give a different set of axioms which assume only the existence of a map ι without specifying it. We leave it to those who like that kind of thing to check that associa- tivity of triple multiplications implies that for any k N, bracketing a k-tuple of ∈ group elements (g1, g2, . -
Ideal Classes and SL 2
IDEAL CLASSES AND SL2 KEITH CONRAD 1. Introduction A standard group action in complex analysis is the action of GL2(C) on the Riemann sphere C [ f1g by linear fractional transformations (M¨obiustransformations): a b az + b (1.1) z = : c d cz + d We need to allow the value 1 since cz + d might be 0. (If that happens, az + b 6= 0 since a b ( c d ) is invertible.) When z = 1, the value of (1.1) is a=c 2 C [ f1g. It is easy to see this action of GL2(C) on the Riemann sphere is transitive (that is, there is one orbit): for every a 2 C, a a − 1 (1.2) 1 = a; 1 1 a a−1 so the orbit of 1 passes through all points. In fact, since ( 1 1 ) has determinant 1, the action of SL2(C) on C [ f1g is transitive. However, the action of SL2(R) on the Riemann sphere is not transitive. The reason is the formula for imaginary parts under a real linear fractional transformation: az + b (ad − bc) Im(z) Im = cz + d jcz + dj2 a b a b when ( c d ) 2 GL2(R). Thus the imaginary parts of z and ( c d )z have the same sign when a b ( c d ) has determinant 1. The action of SL2(R) on the Riemann sphere has three orbits: R [ f1g, the upper half-plane h = fx + iy : y > 0g, and the lower half-plane. To see that the action of SL2(R) on h is transitive, pick x + iy with y > 0. -
A Grothendieck Site Is a Small Category C Equipped with a Grothendieck Topology T
Contents 5 Grothendieck topologies 1 6 Exactness properties 10 7 Geometric morphisms 17 8 Points and Boolean localization 22 5 Grothendieck topologies A Grothendieck site is a small category C equipped with a Grothendieck topology T . A Grothendieck topology T consists of a collec- tion of subfunctors R ⊂ hom( ;U); U 2 C ; called covering sieves, such that the following hold: 1) (base change) If R ⊂ hom( ;U) is covering and f : V ! U is a morphism of C , then f −1(R) = fg : W ! V j f · g 2 Rg is covering for V. 2) (local character) Suppose R;R0 ⊂ hom( ;U) and R is covering. If f −1(R0) is covering for all f : V ! U in R, then R0 is covering. 3) hom( ;U) is covering for all U 2 C . 1 Typically, Grothendieck topologies arise from cov- ering families in sites C having pullbacks. Cover- ing families are sets of maps which generate cov- ering sieves. Suppose that C has pullbacks. A topology T on C consists of families of sets of morphisms ffa : Ua ! Ug; U 2 C ; called covering families, such that 1) Suppose fa : Ua ! U is a covering family and y : V ! U is a morphism of C . Then the set of all V ×U Ua ! V is a covering family for V. 2) Suppose ffa : Ua ! Vg is covering, and fga;b : Wa;b ! Uag is covering for all a. Then the set of composites ga;b fa Wa;b −−! Ua −! U is covering. 3) The singleton set f1 : U ! Ug is covering for each U 2 C . -
On Sheaf Theory
Lectures on Sheaf Theory by C.H. Dowker Tata Institute of Fundamental Research Bombay 1957 Lectures on Sheaf Theory by C.H. Dowker Notes by S.V. Adavi and N. Ramabhadran Tata Institute of Fundamental Research Bombay 1956 Contents 1 Lecture 1 1 2 Lecture 2 5 3 Lecture 3 9 4 Lecture 4 15 5 Lecture 5 21 6 Lecture 6 27 7 Lecture 7 31 8 Lecture 8 35 9 Lecture 9 41 10 Lecture 10 47 11 Lecture 11 55 12 Lecture 12 59 13 Lecture 13 65 14 Lecture 14 73 iii iv Contents 15 Lecture 15 81 16 Lecture 16 87 17 Lecture 17 93 18 Lecture 18 101 19 Lecture 19 107 20 Lecture 20 113 21 Lecture 21 123 22 Lecture 22 129 23 Lecture 23 135 24 Lecture 24 139 25 Lecture 25 143 26 Lecture 26 147 27 Lecture 27 155 28 Lecture 28 161 29 Lecture 29 167 30 Lecture 30 171 31 Lecture 31 177 32 Lecture 32 183 33 Lecture 33 189 Lecture 1 Sheaves. 1 onto Definition. A sheaf S = (S, τ, X) of abelian groups is a map π : S −−−→ X, where S and X are topological spaces, such that 1. π is a local homeomorphism, 2. for each x ∈ X, π−1(x) is an abelian group, 3. addition is continuous. That π is a local homeomorphism means that for each point p ∈ S , there is an open set G with p ∈ G such that π|G maps G homeomorphi- cally onto some open set π(G). -
LECTURE 1. Differentiable Manifolds, Differentiable Maps
LECTURE 1. Differentiable manifolds, differentiable maps Def: topological m-manifold. Locally euclidean, Hausdorff, second-countable space. At each point we have a local chart. (U; h), where h : U ! Rm is a topological embedding (homeomorphism onto its image) and h(U) is open in Rm. Def: differentiable structure. An atlas of class Cr (r ≥ 1 or r = 1 ) for a topological m-manifold M is a collection U of local charts (U; h) satisfying: (i) The domains U of the charts in U define an open cover of M; (ii) If two domains of charts (U; h); (V; k) in U overlap (U \V 6= ;) , then the transition map: k ◦ h−1 : h(U \ V ) ! k(U \ V ) is a Cr diffeomorphism of open sets in Rm. (iii) The atlas U is maximal for property (ii). Def: differentiable map between differentiable manifolds. f : M m ! N n (continuous) is differentiable at p if for any local charts (U; h); (V; k) at p, resp. f(p) with f(U) ⊂ V the map k ◦ f ◦ h−1 = F : h(U) ! k(V ) is m differentiable at x0 = h(p) (as a map from an open subset of R to an open subset of Rn.) f : M ! N (continuous) is of class Cr if for any charts (U; h); (V; k) on M resp. N with f(U) ⊂ V , the composition k ◦ f ◦ h−1 : h(U) ! k(V ) is of class Cr (between open subsets of Rm, resp. Rn.) f : M ! N of class Cr is an immersion if, for any local charts (U; h); (V; k) as above (for M, resp. -
When Is a Local Homeomorphism a Semicovering Map? in This Section, We Obtained Some Conditions Under Which a Local Homeomorphism Is a Semicovering Map
When Is a Local Homeomorphism a Semicovering Map? Majid Kowkabia, Behrooz Mashayekhya,∗, Hamid Torabia aDepartment of Pure Mathematics, Center of Excellence in Analysis on Algebraic Structures, Ferdowsi University of Mashhad, P.O.Box 1159-91775, Mashhad, Iran Abstract In this paper, by reviewing the concept of semicovering maps, we present some conditions under which a local homeomorphism becomes a semicovering map. We also obtain some conditions under which a local homeomorphism is a covering map. Keywords: local homeomorphism, fundamental group, covering map, semicovering map. 2010 MSC: 57M10, 57M12, 57M05 1. Introduction It is well-known that every covering map is a local homeomorphism. The converse seems an interesting question that when a local homeomorphism is a covering map (see [4], [8], [6].) Recently, Brazas [2, Definition 3.1] generalized the concept of covering map by the phrase “A semicovering map is a local homeomorphism with continuous lifting of paths and homotopies”. Note that a map p : Y → X has continuous lifting of paths if ρp : (ρY )y → (ρX)p(y) defined by ρp(α) = p ◦ α is a homeomorphism for all y ∈ Y, where (ρY )y = {α : [0, 1] → Y |α(0) = y}. Also, a map p : Y → X has continuous lifting of homotopies if Φp : (ΦY )y → (ΦX)p(y) defined by Φp(φ)= p ◦ φ is a homeomorphism for all y ∈ Y , where elements of (ΦY )y are endpoint preserving homotopies of paths starting at y. It is easy to see that any covering map is a semicovering. qtop The quasitopological fundamental group π1 (X, x) is the quotient space of the loop space Ω(X, x) equipped with the compact-open topology with respect to the function Ω(X, x) → π1(X, x) identifying path components (see [1]). -
Chapter 9: Group Actions
Chapter 9: Group actions Matthew Macauley Department of Mathematical Sciences Clemson University http://www.math.clemson.edu/~macaule/ Math 4120, Spring 2014 M. Macauley (Clemson) Chapter 9: Group actions Math 4120, Spring 2014 1 / 27 Overview Intuitively, a group action occurs when a group G \naturally permutes" a set S of states. For example: The \Rubik's cube group" consists of the 4:3 × 1019 actions that permutated the 4:3 × 1019 configurations of the cube. The group D4 consists of the 8 symmetries of the square. These symmetries are actions that permuted the 8 configurations of the square. Group actions help us understand the interplay between the actual group of actions and sets of objects that they \rearrange." There are many other examples of groups that \act on" sets of objects. We will see examples when the group and the set have different sizes. There is a rich theory of group actions, and it can be used to prove many deep results in group theory. M. Macauley (Clemson) Chapter 9: Group actions Math 4120, Spring 2014 2 / 27 Actions vs. configurations 1 2 The group D can be thought of as the 8 symmetries of the square: 4 4 3 There is a subtle but important distinction to make, between the actual 8 symmetries of the square, and the 8 configurations. For example, the 8 symmetries (alternatively, \actions") can be thought of as e; r; r 2; r 3; f ; rf ; r 2f ; r 3f : The 8 configurations (or states) of the square are the following: 1 2 4 1 3 4 2 3 2 1 3 2 4 3 1 4 4 3 3 2 2 1 1 4 3 4 4 1 1 2 2 3 When we were just learning about groups, we made an action diagram.