City University of New York (CUNY) CUNY Academic Works All Dissertations, Theses, and Capstone Projects Dissertations, Theses, and Capstone Projects 6-2020 Symmetric Rigidity for Circle Endomorphisms with Bounded Geometry and Their Dual Maps John Adamski The Graduate Center, City University of New York How does access to this work benefit ou?y Let us know! More information about this work at: https://academicworks.cuny.edu/gc_etds/3790 Discover additional works at: https://academicworks.cuny.edu This work is made publicly available by the City University of New York (CUNY). Contact: [email protected] Symmetric rigidity for circle endomorphisms with bounded geometry and their dual maps by John Adamski A dissertation submitted to the Graduate Faculty in Mathematics in partial fulfillment of the requirements for the degree of Doctor of Philosophy, The City University of New York. 2020 ii c 2020 John Adamski All Rights Reserved iii This manuscript has been read and accepted for the Graduate Faculty in Mathematics in satisfaction of the dissertation requirements for the degree of Doctor of Philosophy. (required signature) Date Chair of Examining Committee (required signature) Date Executive Officer Dr. Yunping Jiang Dr. Linda Keen Dr. Frederick Gardiner Dr. Sudeb Mitra, Dr. Zhe Wang Supervisory Committee THE CITY UNIVERSITY OF NEW YORK iv Abstract Symmetric rigidity for circle endomorphisms with bounded geometry and their dual maps by John Adamski Advisor: Dr. Yunping Jiang Let f be a circle endomorphism of degree d ≥ 2 that generates a se- quence of Markov partitions that either has bounded nearby geometry and bounded geometry, or else just has bounded geometry, with respect to nor- malized Lebesgue measure. We define the dual symbolic space Σ∗ and the dual circle endomorphism f ∗ = h~ ◦ f ◦ h−1, which is topologically conjugate to f. We describe some properties of the topological conjugacy h~. We also describe an algorithm for generating arbitrary circle endomorphisms f with bounded geometry that preserve Lebesgue measure and their corresponding dual circle endomorphisms f ∗ as well as the conjugacy h~, and implement it using MATLAB. We use the property of bounded geometry to define a convergent Mar- v tingale on Σ∗, and apply the study of such Martingales to obtain a rigidity theorem. Suppose f and g are two circle endomorphisms of the same degree d ≥ 2 such that each has bounded geometry and each preserves the normal- ized Lebesgue probability measure. Suppose that f and g are symmetrically conjugate. That is, g = h ◦ f ◦ h−1 and h is a symmetric circle homeomor- phism. We define a property called locally constant limit of Martingale, and show that if f has this property then f = g. Acknowledgements Thank you to my advisor, Professor Yunping Jiang, for your steady guidance, abundant wisdom, and infinite patience. For the many discussions on Friday afternoons, when I left your office with more ideas than I had when I entered, I will be forever grateful. Thank you to Professor Zhe Wang and Professor Yunchun Hu for the generosity you showed me with your time and your thoughts. Thank you to Professor Fred Gardiner and Professor Linda Keen for sharing your passion and your knowledge, taking interest in my work and progress, and for being bottomless sources of encouragement. Thank you to Professor Tao Chen for all of your help, your cheery disposition, and your flexibility with scheduling over the years. Thank you to Professor Sudeb Mitra for putting front and center the joy of methematics and having a good sense of humor, both inside and outside of the classroom. Thank you to my friends who helped in so many ways big and small, vi vii providing the right balance of encouragement and distraction: Eli Amzallag, Dave and Maria Anderson, Jess Bogwicz, Hina Dar, Corey J. Feldman, Ryan and Julia Ford, Nick Hundley, Megan Moskop, Nick and Katie Nacca, Anna Padilla, Jon and Marlee Patrizio, Shilpa Ray, Dave Schnurman, Leigh Sugar, Rylan Summit, Warren Tai, and Tara Thomas. Thank you to my family. And a special thank you to my mom and dad. I love you both. Contents 1 Quasiconformal and quasisymmetric maps 1 1.1 Conformal maps . .1 1.2 Quasi-conformal maps . .7 1.3 Quasisymmetric and uniformly quasisymmetric maps . 11 2 Circle endomorphisms 13 2.1 Circle homeomorphisms and endomorphisms . 13 2.2 Symbolic space and topological representation . 22 2.3 Bounded geometry and bounded nearby geometry . 27 2.4 Measures with bounded geometry and bounded nearby geometry 49 3 Invariant measures 52 3.1 Borel measures . 52 3.2 Invariant measures and ergodicity . 60 viii CONTENTS ix 3.3 Invariant measures on T with bounded geometry and bounded nearby geometry . 70 3.4 Constructing examples of distribution functions for measures with bounded geometry preserved by q2 ............. 74 4 The dual circle map 92 4.1 Equivalence of partitions and maps . 92 4.2 Dual partitions, dual maps, and dual measures . 95 4.3 Properties of the dual homeomorphism . 102 4.4 Dual symbolic space . 107 5 Martingales 114 5.1 Definition and convergence theorem . 114 5.2 Locally constant limits and finite martingales . 127 5.3 A symmetric rigidity theorem assuming locally constant limit of martingale . 137 6 Appendix 140 6.1 Bernoulli measure . 140 6.2 Bounded geometry measure . 141 6.3 Dual measure and dual endomorphism . 146 CONTENTS x Bibliography 153 List of Figures 1.1 Conformally mapping a quadrilateral onto a rectangle . 10 2.1 On the left is the graph of the ternary Cantor function on [0; 1]. On the right is the graph of F (x) = x + C1=3(x) on [0; 1]. Both functions map the same sets of measure 0 to sets of measure 1. 19 2.2 f −1(1) cuts T into d − 1 closed arcs. 22 2.3 Gk(x) for d =2........................... 32 2.4 One possible way that x − t; x; and x + t may be contained in partitions ηN−1;f ; ηN;f ; and ηN+N1;f . The corresponding figure for how H(x − t);H(x); and H(x + t) would be contained in partitions ηN−1;g; ηN;g; and ηN+N1;g is homeomorphic. 35 2.5 The circle endomorphism Fα shown acting on R=Z. Here α = :8. The map Fα has bounded geometry but it is not uniformly quasisymmetric. 44 xi LIST OF FIGURES xii 2.6 The circle homeomorphism Hα shown acting on R=Z. It is not −1 quasisymmetric. Fα = Hα ◦ Q2 ◦ Hα ............... 45 2.7 The circle endomorphism F~ shown acting on R=Z. The map F~ is uniformly quasisymmetric. Thus is has bounded geometry and bounded nearby geometry. 46 2.8 The circle homeomorphism H~ shown acting on R=Z. It is ~ ~ ~ −1 quasisymmetric. F = H ◦ Q2 ◦ H ................ 47 3.1 For all f 2 C(X) and for all open intervals I ⊆ [0; 1], the set R of all measures µ 2 M(X) such that X f dµ 2 I belongs to the weak∗ topology on M(X). The weak∗ topology on M(X) is the smallest topology satisfying this condition. 53 3.2 On the left are the distribution functions h(x) = µ([0; x]) for two Bernoulli measures µ. The top Bernoulli measure has α = :2 and the bottom Bernoulli measure has α = :8. On the −1 right are the corresponding circle endomorphisms f = h◦q2 ◦h . 78 3.3 On the left are the distribution functions h(x) = µ([0; x]) for two Bernoulli measures µ. The top Bernoulli measure has α = :4 and the bottom Bernoulli measure has α = :9. On the −1 right are the corresponding circle endomorphisms f = h◦q2 ◦h . 79 LIST OF FIGURES xiii 3.4 The interval I!n 2 ηn and its subinterval I!n0 2 ηn+1, along with their preimages belonging to ηn+1 and ηn+2, respectively. 86 3.5 On the left are the distribution functions h(x) = µ([0; x]) for three arbitrary measures µ that are invariant with respect to the doubing map q2, and that have bounded geometry with C = :1. On the right are the corresponding circle endomor- −1 phisms f = h ◦ q2 ◦ h ...................... 91 4.1 The lifts F0, F1, and F2...................... 94 4.2 For d = 2 the intervals of ηn are labeled as n-digit binary numbers, increasing left to right, from 0 to 2n+1 − 1. 96 ∗ 4.3 Shuffling intervals of ηn to construct ηn.............. 98 ∗ 4.4 Topologically conjugate circle endomorphisms qd; f; and f (the dual circle endomorphism of f). 100 4.5 Distribution functions h (left, blue) and dual distribution func- tions h∗ (left, red), and their corresponding circle endomor- −1 ∗ ∗ ∗ −1 phisms f = h ◦ q2 ◦ h (right, blue) and f = h ◦ q2 ◦ (h ) (right, red). On the left in black is the dual homeomorphism h~ = h∗ ◦ h−1............................ 101 LIST OF FIGURES xiv 1 ~ 4.6 The increasing sequence famgm=1 of fixed points of h, con- structed with n =2. ....................... 105 4.7 Two directed d-nary trees (d=2). The dotted tree shows the action of σ on left-cylinders of Σ, and the solid tree shows the action of σ∗ on right-cylinders of Σ∗............... 110 5.1 Finite Martingale of length n0 = 3 and C = 4. The top left shows the lengths of the intervals belonging to ηn and the top right shows the values of Xn on intervals belonging to ηn, for n = 1; 2; 3; 4. The bottom left shows the distribution function h and the bottom right shows the circle endomorphism f, with −1 f = h ◦ q2 ◦ h . The circle endomorphism f has bounded geometry but does not have bounded nearby geometry, thus it is not uniformly quasisymmetric.