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Quasiconvexity in Word-Hyperbolic Coxeter Groups

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Quasiconvexity in Word-Hyperbolic Coxeter Groups Quasiconvexity in Word-Hyperbolic Coxeter Groups Adam Simon Levine Mathematics Department Harvard University April 3, 2005 Abstract In Chapter 1, I outline many of the basic properties of word-hyperbolic groups and give an introduction to CAT(κ) spaces. In Chapter 2, I prove a number of important theorems about Coxeter groups, including the Exchange Condition and Tits's solution to the word problem. I also discuss Moussong's theorem on which Coxeter groups are word-hyperbolic. Finally, in Chapter 3, I present some original research showing that certain subgroups of certain word-hyperbolic Coxeter groups are quasiconvex. Acknowledgements I am deeply grateful to the following people for their assistance with this thesis: To my advisor, Prof. Dylan Thurston, for guiding me through the thesis-writing process, always taking the time to answer my questions, and constantly challenging me to learn more. To Profs. Patrick Bahls and Kim Whittlesey, for supervising my summer research at the University of Illinois at Urbana-Champaign and sparking my interest in geometric group theory. To Mark Lipson, for his meticulous proofreading and his pages of detailed suggestions and corrections. To Peter Green and David Escott, for their helpful comments. To my roommates, for putting up with my math puns for four years. To Amy Pasternack, for her late-night cookie deliveries to the Science Center computer lab. To my brother Ben, for being a good sport during my lengthy explanations of Coxeter groups, and for being a dear friend. To my parents, for their helpful advice on how to make my thesis comprehensible to laypeople, their valiant efforts to explain to friends and relatives exactly what I study, and their constant love and support. And finally, to my high school calculus teacher, Mr. Robert Arrigo, for instilling in me a lifelong love of mathematics. Contents List of Figures 2 Introduction 3 1 The Theory of Word-Hyperbolic Groups 11 1.1 δ-Hyperbolic Metric Spaces and Quasi-Isometries . 11 1.2 Word-Hyperbolic Groups . 15 1.3 The Rips Complex and Finite Presentations . 18 1.4 Quasiconvexity . 22 1.5 Spaces of Non-Positive Curvature . 25 2 Coxeter Groups 28 2.1 The Geometric Representation . 29 2.2 The Exchange and Deletion Conditions . 32 2.3 Discrete Reflection Groups . 36 2.4 The Word Problem in Coxeter Groups . 41 2.5 Parabolic Subgroups . 43 2.6 Word-Hyperbolic Coxeter Groups . 45 3 Original Results 50 Poetic Conclusion 56 Bibliography 58 1 List of Figures 1 Reflections through lines meeting at 60◦. 4 2 Cayley graph for Coxeter group generated by a, b, and c, subject to the rules aba = bab, bcb = cbc, and ac = ca. 5 3 Cayley graph for Coxeter group generated by a, b, and c, subject to the rules aba = bab, bcb = cbc, and aca = cac. 6 4 The hyperboloid (left) and Poincar´e (right) models of the hyperbolic plane. 7 5 Cayley graph for the (2; 4; 6) triangle group, embedded in the Poincar´e model of the hyperbolic plane H2. 8 6 M.C. Escher's woodcut Circular Limit I [1, p. 319]. 10 7 A δ-slim triangle. 12 8 Schematic of the proof of Theorem 1.4. 14 9 The two cases in the proof of Lemma 1.13. 20 10 A k-quasiconvex subspace Y . 22 2 2 11 The Cayley graphs A(Z ) (left) and B(Z ) (right) for Example 1.16. 23 C C 12 Schematic of the proof of Theorem 1.18. 24 13 The positions of the roots v:es in the proof of Lemma 2.5 when ms;t = 5, along with the fundamental domain K defined at the end of the section. 34 14 Coxeter diagrams for the irreducible finite and affine Coxeter groups. 40 15 The Coxeter cell C(W ) and block B(W ) for the (2; 2; 3) triangle group. 47 16 Schematic of the proof of Theorem 3.2. 53 17 The two types of cancellation diagrams in Proposition 3.4. 55 2 Introduction Given a finite set of letters, consider the set of words | not English words, but simply finite sequences of letters | that can be written with those letters. We define two operations: Whenever a letter appears twice consecutively, we may delete both occurrences. Like- • wise, we may insert two consecutive, identical letters into a word at any point. For each pair of letters (say, a and b), choose one of the following rules: ab = ba, • aba = bab, abab = baba, and so on. That is, if we see a sequence of the form aba , · · · we may replace it with bab (where both sequences are of the specified length), and vice versa. These rules need· · ·not be the same for different pairs of letters; we could say, for instance, that ab = ba, aca = cac, and bcbc = cbcb. We may also choose not to impose any such rule on particular pairs of letters. Two words are said to be equivalent if it is possible to get from one to the other using these two operations. The set of all classes of equivalent words is called a Coxeter group (named for the geometer H.S.M. Coxeter, who studied this construction starting in the 1930s). For instance, if the rules are ab = ba, aca = cac, and bcbc = cbcb as above, then the words abac, aabc, baac, and bc are all equivalent, so they represent the same element of the Coxeter group. They are not equivalent, however, to the word cb, which thus represents a different element of the Coxeter group.1 As an example, let the set of letters (also called generators) consist of a and b, and suppose that aba = bab. The elements of the Coxeter group can be written as a, b, ab, ba, aba (which is the same as bab), and the so-called empty word, a word of zero letters, often denoted by 1. Any longer word can be reduced to one of these six possibilities. As another example, the group generated by a, b, and c, with the rules aba = bab, bcb = cbc, and ac = ca, has 24 distinct elements. On the other hand, if the rules are aba = bab, bcb = cbc, and aca = cac, then the group contains infinitely many distinct elements, for the words abc, abcabc, abcabcabc, abcabcabcabc, etc. all represent different elements. Many Coxeter groups occur naturally as so-called reflection groups, sets of transforma- tions of a space that can be realized as the composition of a given set of reflections. For instance, consider two lines in the plane, labelled La and Lb, that intersect at an angle of 60◦. 1Formally, in order to call this set a group, we need to define a law of composition. However, as this Introduction is intended for a general audience, we shall use this colloquial description. A rigorous definition of Coxeter groups will appear in Chapter 2. 3 Lb La Figure 1: Reflections through lines meeting at 60◦. Take any figure, such as the upper right letter R shown in Figure 1, and reflect it across the line La to obtain the upside-down R in the lower-right corner. Next, reflect the new figure across Lb to obtain the R in the upper-left corner, and then reflect that down across La to the lower-left backward R. Next, starting with the original figure again, reflect first across Lb, then across La, then across Lb again, and note that the final image is exactly the same. In other words, if a and b represent reflections across La and Lb, respectively, then we obtain the rule aba = bab. Also, performing the same reflection twice in a row leaves every point fixed, so we may insert or delete pairs of consecutive, identical reflections into any sequence without consequence. Thus, we can say that the set of transformations of the plane that are compositions of the reflections a and b is a Coxeter group. More generally, if La and Lb intersect instead at an angle of 180◦=n, where n is an integer greater than 1, we obtain a Coxeter group with the rule aba = bab : · · · · · · n letters n letters One way to visualize a Coxeter group| {zgeometrically} | {z } is the Cayley graph. This is a graph (a network of vertices connected by edges) with one vertex for every distinct element of the group and with edges labelled with the different letters, one of each type at each ver- tex. Words written with the generating letters correspond to paths emanating from a fixed starting point, and two words represent the same element of the group if and only if the corresponding paths end at the same point. For example, the Cayley graph for the Coxeter group generated by a and b with the rule that ab = ba is a square with its sides alternately labelled a and b. If aba = bab, the graph is a hexagon; if abab = baba, an octagon; and so on. Assume that the polygons are regular; that is, all the sides and all the angles are equal.2 With more than two letters, the graph 2Formally, the angle measures are a property not of the graph itself but of the embedding of the graph into a larger space, in this case Rn. To avoid having to introduce the concept of an abstractly defined graph, 4 Figure 2: Cayley graph for Coxeter group generated by a, b, and c, subject to the rules aba = bab, bcb = cbc, and ac = ca. can be formed by gluing together polygons of the appropriate types in such a way that there is one polygon of each type at every vertex.
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