For any element A of the modular group PSL(2,Z), it follows from work of Bavard that scl(A) is greater than or equal to rot(A)/2, where scl denotes stable commutator length and rot denotes the rotation quasimorphism. Sometimes this bound is sharp, and sometimes it is not. We study for which elements A in PSL(2,Z) the rotation quasimorphism is extremal in the sense that scl(A)=rot(A)/2. First, we explain how to compute stable commutator length in the modular group, which allows us to experimentally determine whether the rotation quasimorphism is extremal for a given A. Then we describe some experimental results based on data from these computations.

Our main theorem is the following: for any element of the modular group, the product of this element with a sufficiently large power of a parabolic element is an element for which the rotation quasimorphism is extremal. We prove this theorem using a geometric approach. It follows from work of Calegari that the rotation quasimorphism is extremal for a hyperbolic element of the modular group if and only if the corresponding geodesic on the modular surface virtually bounds an immersed surface. We explicitly construct immersed orbifolds in the modular surface, thereby verifying this geometric condition for appropriate geodesics. Our results generalize to the 3-strand braid group and to arbitrary Hecke triangle groups.

}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Calegari, Danny C.}, } @phdthesis{10.7907/082X-F229, author = {Gadre, Vaibhav S.}, title = {Dynamics of Non-Classical Interval Exchanges}, school = {California Institute of Technology}, year = {2010}, doi = {10.7907/082X-F229}, url = {https://resolver.caltech.edu/CaltechTHESIS:04212010-181932550}, abstract = {Train tracks with a single vertex are a generalization of interval exchange maps. Here, we consider non-classical interval exchanges: complete train tracks with a single vertex. These can be studied as a dynamical system by considering Rauzy induction in this context. This gives a refinement process on the parameter space similar to Kerckhoff’s simplicial systems. We show that the refinement process gives an expansion that has a key dynamical property called uniform distortion. We use uniform distortion to prove normality of the expansion. Consequently, we prove an analog of Keane’s conjecture: almost every non-classical interval exchange is uniquely ergodic. In the concluding chapter, we state an application of the main results of the thesis to a question about harmonic measures on the Thurston boundary of Teichmuller space coming from finitely supported random walks on the mapping class group.}, address = {1200 East California Boulevard, Pasadena, California 91125}, month = {July}, } @phdthesis{10.7907/J566-2537, author = {Zhuang, Dongping}, title = {A Geometric Study of Commutator Subgroups}, school = {California Institute of Technology}, year = {2009}, doi = {10.7907/J566-2537}, url = {https://resolver.caltech.edu/CaltechETD:etd-05152009-115934}, abstract = {Let G be a group and G’ its commutator subgroup. Commutator length (cl) and stable commutator length (scl) are naturally defined concepts for elements of G’. We study cl and scl for two classes of groups. First, we compute scl in generalized Thompson’s groups and their central extensions. As a consequence, we find examples of finitely presented groups in which scl takes irrational (in fact, transcendental) values. Second, we study large scale geometry of the Cayley graph of a commutator subgroup with respect to the canonical generating set of all commutators. When G is a non-elementary hyperbolic group, we prove that, for any n, there exists a quasi-isometrically embedded, dimension n integral lattice in this graph. Thus this graph is not hyperbolic, has infinite asymptotic dimension, and has only one end. For a general finitely presented group, we show that this graph is large scale simply connected.

}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Calegari, Danny C.}, } @phdthesis{10.7907/290Y-BY53, author = {Venzke, Rupert William}, title = {Braid Forcing, Hyperbolic Geometry, and Pseudo-Anosov Sequences of Low Entropy}, school = {California Institute of Technology}, year = {2008}, doi = {10.7907/290Y-BY53}, url = {https://resolver.caltech.edu/CaltechETD:etd-05292008-085545}, abstract = {We view braids as automorphisms of punctured disks and define a partial order on pseudo-Anosov braids called the “forcing order”. The order measures whether one automorphism induces another given automorphism on the surface. Pseudo-Anosov growth rate decreases relative to the order and appears to give a good measure of braid complexity. Unfortunately it appears difficult computationally to determine explicitly the partial order structure by hand. We use several computer algorithms to study the bottom part of the partial order when the number of braid strands is fixed. From the algorithms, we build sequences of low entropy pseudo-Anosov n-strand braids that are minimal in the sense that they do not force any other pseudo-Anosov braids on the same number of strands. The sequences are an extension of work done by Hironaka and Kin, and we conjecture the sequences to achieve minimal entropy among certain non-trivial classes of braids. In general, the lowest entropy pseudo-Anosov braids appear to have mapping tori that come from Dehn surgery on very low volume hyperbolic 3-manifolds, and we begin to analyze the relation between entropy and hyperbolic volume. Moreover, the low-growth families contain non-trivial low-growth families of horseshoe braids and we proceed to study dynamics of the horseshoe map as well.

}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Calegari, Danny C.}, } @phdthesis{10.7907/Q16J-V757, author = {Pelayo, Roberto Carlos}, title = {Diameter Bounds on the Complex of Minimal Genus Seifert Surfaces for Hyperbolic Knot}, school = {California Institute of Technology}, year = {2007}, doi = {10.7907/Q16J-V757}, url = {https://resolver.caltech.edu/CaltechETD:etd-06042007-015951}, abstract = {Given a link L in the 3-sphere, one can build simplicial complexes MS(L) and IS(L), called the Kakimizu complexes. These complexes have isotopy classes of minimal genus and incompressible Seifert surfaces for L as their vertex sets and have simplicial structures defined via a disjointness property. The Kakimizu complexes enjoy many topological properties and are conjectured to be contractible. Following the work of Gabai on sutured manifolds and Murasugi sums, MS(L) and IS(L) have been classified for various classes of links. This thesis focuses on hyperbolic knots; using minimal surface representatives and Kakimizu’s formulation of the path-metric on MS(K), we are able to bound the diameter of this complex in terms of only the genus of the knot. The techniques of this paper are also generalized to one-cusped manifolds with a preferred relative homology class.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Calegari, Danny C.}, } @phdthesis{10.7907/35GG-W072, author = {Mack, Thomas Patrick}, title = {Quasiconvex Subgroups and Nets in Hyperbolic Groups}, school = {California Institute of Technology}, year = {2006}, doi = {10.7907/35GG-W072}, url = {https://resolver.caltech.edu/CaltechETD:etd-06052006-141903}, abstract = {Consider a hyperbolic group G and a quasiconvex subgroup H of G with [G:H] infinite. We construct a set-theoretic section s:G/H -> G of the quotient map (of sets) G -> G/H such that s(G/H) is a net in G; that is, any element of G is a bounded distance from s(G/H). This set arises naturally as a set of points minimizing word-length in each fixed coset gH. The left action of G on G/H induces an action on s(G/H), which we use to prove that H contains no infinite subgroups normal in G.

}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Calegari, Danny C.}, }