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Homotopy Theory of Gauge Groups Over 4-Manifolds

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Homotopy Theory of Gauge Groups Over 4-Manifolds UNIVERSITY OF SOUTHAMPTON Homotopy theory of gauge groups over 4-manifolds by Tseleung So A thesis submitted in partial fulfillment for the degree of Doctor of Philosophy in the Faculty of Social, Human and Mathematical Sciences Department of Mathematics July 2018 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF SOCIAL, HUMAN AND MATHEMATICAL SCIENCES Department of Mathematics Doctor of Philosophy HOMOTOPY THEORY OF GAUGE GROUPS OVER 4-MANIFOLDS by Tseleung So Given a principal G-bundle P over a space X, the gauge group (P ) of P is the topolo- G gical group of G-equivariant automorphisms of P which fix X. The study of gauge groups has a deep connection to topics in algebraic geometry and the topology of 4-manifolds. Topologists have been studying the topology of gauge groups of principal G-bundles over 4-manifolds for a long time. In this thesis, we investigate the homotopy types of gauge groups when X is an orientable, connected, closed 4-manifold. In particular, we study the homotopy types of gauge groups when X is a non-simply-connected 4-manifold or a simply-connected non-spin 4-manifold. Furthermore, we calculate the orders of the Samelson products on low rank Lie groups, which help determine the classification of gauge groups over S4. Contents Declaration of Authorship vii Acknowledgements ix 1 Introduction1 2 A review of homotopy theory5 2.1 Basic notions..................................5 2.2 H-spaces and co-H-spaces...........................7 2.3 Fibrations and cofibrations.......................... 10 3 Principal G-bundles and gauge groups 21 3.1 Fiber bundles, principal G-bundles and classifying spaces.......... 21 3.2 Gauge groups.................................. 24 4 Homotopy decomposition of Lie groups 29 4.1 Topological localization............................ 29 4.2 Localization of Lie groups........................... 35 5 Gauge groups and Samelson products 39 6 Homotopy types of gauge groups over non-simply-connected closed 4-manifolds 43 7 The odd primary order of the commutator on low rank Lie groups 65 8 Homotopy types of SU(n)-gauge groups over non-spin 4-manifolds 83 Bibliography 103 v Academic thesis: Declaration of authorship I, Tseleung So, declare that this thesis and the work presented in it are my own and has been generated by me as the result of my own original research. Homotopy theory of gauge groups over 4-manifolds I confirm that: 1. This work was done wholly or mainly while in candidature for a research degree at this University; 2. Where any part of this thesis has previously been submitted for a degree or any other qualification at this University or any other institution, this has been clearly stated; 3. Where I have consulted the published work of others, this is always clearly attributed; 4. Where I have quoted from the work of others, the source is always given. With the exception of such quotations, this thesis is entirely my own work; 5. I have acknowledged all main sources of help; 6. Where the thesis is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself; 7. Either none of this work has been published before submission, or parts of this work have been published as: Signed: ....................................................................... Dated: ........................................................................ vii Acknowledgements First of all, I would like to thank my family who always support my decision and back me up. Without their support I cannot complete this project. I would like to thank my supervisor Prof Stephen Theriault for his great help in my PhD life. He gave clear guidance on my thesis and papers and spent much time proofreading them, for which he should take most of the credit. Knowing my English is not good, in our meetings he repeated his explanation with patience to ensure I fully understood it and he did a lot to improve my presentation and writing skills. Furthermore, he gave useful career advice and helped me apply for postdoctoral jobs. I was so lucky to have Stephen, who is nice, patient and helpful, as my supervisor. Thank my examiners Prof Jie Wu and Prof Nansen Petrosyan for reviewing my thesis and giving a lot of useful comments. Living in Southampton in the last last four years was an amazing and memorable ex- perience to me. There are too many people on my thank-you list to put down in the acknowledgement, including Jelena, Vesna, Bernhard, Shizuo and Mikiko, Mike, Dave, Tom Harris, Joe, Charles, Matt, Ingrid, Ariana, Robin, Hector, Ana, Ashley, Semih, Tyrone, Abi, Xin, Fabio, James, Hollis, Motiejus, Conrad, Naomi, Laurie, Tom Brown, Megan, Simon, Holly, Vlad, Jiawen, Kiko, Mariam, Donya, Jamie, Ilaria, two Martinas, William and Jing, Andy, Jason, Heyi, Ace, Cherry and Louis. In particular, I thank all current and previous people in Room 7007 and 8035 for providing a friendly, supportive and enjoyable atmosphere, thank the algebraic topologists for your company in many happy trips, thank Mike, Matt, Ingrid and Hector for discussing mathematics with me, and thank Mike and Hector for teaching me atypical English culture and knowledge. I also thank Prof Conan Leung, Prof Haizhong Li and Prof Tom Wan for helping my PhD applications, and my friends in Hong Kong for your friendship. I especially thank Alis, Liu-wong, Cyril, Chara, Man-sai, Sarah and Ryan for your support and encouragement in my difficult times. Writing this acknowledgement brings back a lot of memories. Now a verse comes to my head: “ ‘Not by might nor by power, but by My Spirit’, says the Lord of hosts.” (Zechariah 4:6) I believe it was my Lord who led me to achieve this. Thank you. ix To my parents and to the memory of my grandmother. xi Chapter 1 Introduction Gauge groups arise naturally in physics. In a physical theory, a local coordinate frame at each point is represented by an element of a Lie group G. A gauge group is a collection of local symmetries, which are continuous pointwise transformations of these local bases. In mathematics, a gauge group (P ) associated to a principal G-bundle P over X is the G topological group of G-equivariant automorphisms of P which fix X. The study of gauge groups has a deep connection with topology and geometry, especially within the study of 4-manifolds, which is one of the most central and difficult topics in modern mathematics. For example, Donaldson applied SU(2)-gauge theory to obtain his groundbreaking results on the classification of smooth 4-manifolds, for which he won the Fields Medal. Suppose X is an orientable, connected, closed 4-manifold and G is a simple, simply- connected, compact Lie group. Every principal G-bundle over X is classified by its second Chern class k Z. Denote its associated gauge group by k(X). When X is a 2 G simply-connected spin 4-manifold, many cases of the homotopy types of gauge groups of X have been investigated over the last twenty years. For examples, there are 6 distinct homotopy types of (X)’s for G = SU(2) [Kon91], and 8 distinct homotopy Gk types for G = SU(3) [HK06]. When localized rationally or at any odd prime, there are 16 distinct homotopy types for G = SU(5) [The15] and 8 distinct homotopy types for G = Sp(2) [The10a]. However, many problems still remain unsolved. Compared to the extensive work on the spin 4-manifold case, only two cases of (X) have been studied Gk for non-spin X, which are the SU(2)- and SU(3)-cases [KT96, The12]. Furthermore, very little is known about (X) when X is a non-simply-connected 4-manifold due to Gk the complexity of the topology of 4-manifolds. 1 2 Chapter 1 Introduction My Ph.D. research includes three interrelated projects: (1) the homotopy types of gauge groups over non-simply-connected 4-manifolds, (2) calculating the orders of the Samelson products on Lie groups and (3) the homotopy types of gauge groups over non-spin 4- manifolds, of which my findings are summarized in my three articles [So18a, So18c, So18b] in Chapter 6 to 8. In the first project, I developed a homotopy decomposition of gauge groups based on ideas in [Sut92, The10b] that enabled me to decompose the gauge groups over certain non-simply-connected 4-manifolds. Theorem A. Let X be an orientable, smooth, connected, closed 4-manifold and let G m r be a simple, simply-connected, compact Lie group. If π1(X) = Z∗ or Z=p Z for some odd prime p, then we have (X) (X0) “loop spaces” on G; Gk 'Gk × 4 2 m n rj where X0 is S or CP . If π1(X) = (Z∗ ( Z=p Z)) for odd primes pj, then we ∗ ∗j=1 j have 2d 2 (X) Ω G (X0) “loop spaces” on G Gk × 'Gk × where d is some positive integer.Y 4 2 The space X0 is S or CP depending on whether the suspension of the attaching map of the 4-cell in X is null homotopic or not. Also, the term “loop spaces” refer both to iterated based loop spaces ΩG; Ω2G and Ω3G and modular loop spaces ΩG pr and Ω2G pr , f g f g where G pr is the homotopy fiber of the pr-power map on G. Explicit decompositions f g are stated in Chapter 6. 4 2 By TheoremA the homotopy types of k(X) are determined by k(S ) and k(CP ). G G G 4 2 Combining the known results of k(S ) and k(CP ) [HK06, KK18, KTT17, Kon91, G G The10a, The15, The17, KT96, The12] gives the following partial classification of (X).
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