## Colloquium 2016-2017

## April 21, Dhruba R. Adhikari, Kennesaw State University

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Title: Maximal Monotonicity and Existence of Nonzero Solutions

Abstract: The theory of maximal monotone operators in Banach spaces

plays an important role in the solvability of a large class of partial differential equations, particularly the ones in divergence form. The talk will begin with an introduction to maximal monotone operators in Hilbert spaces. An extension of the notion to Banach spaces will then be discussed. Among others, examples of maximal monotone operators which appear as the subdifferentials of certain convex functions will be given. Finally, an existence theorem for nonzero solutions of operator equations in Banach spaces will be presented.

## April 14, Howard Weiss, Georgia Tech

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Title: Antibiotic Cycling: A cautionary tail.

Abstract: Antibiotics have greatly reduced morbidity and mortality from infectious diseases. Although antibiotic resistance is not a new problem, it breadth now constitutes a significant threat to human health. One strategy to help combat resistance is to find novel ways of using obsolete antibiotics. For strains of E. coli, pairs of antibiotics have been found where evolution of resistance to one increases, sometimes significantly, sensitivity to the other. These researchers have proposed cycling such pairs to treat infections. Similar strategies are being investigated to treat cancer.

Using systems of ODEs, we model several possible treatment protocols using pairs and triples of such antibiotics, and investigate the speed of ascent of multiply resistant mutants. Rapid ascent would doom this strategy. This is joint work with Klas Udekwu (Stockholm University).

## April 7, Balázs Strenner, Georgia Tech

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Title: Number-theoretic aspects of surface homeomorphisms.

Abstract: The talk will be a survey of results connecting surface homeomorphisms with number theory. The starting point of these connections is the fact that many surface homeomorphism have an associated stretch factor which is an algebraic integer. I will mention several results where a purely number theoretic property of the stretch factor (e.g. the location of its Galois conjugates) translates to a purely topological property of the homeomorphism. The talk will not assume much background in either topology or number theory, so it should be accessible to a broad audience.

## March 24, Stephen Odaibo, Medical Associates Clinic in Dubuque, Iowa

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Title: From Math Major to Physician Scientist and Back: A How-to Guide.

Abstract: So exactly how does a math major become a practicing physician with a steady interest in scientific problem-solving? And why is that a good thing? In this talk, I will provide my answers to both questions. Also, I will briefly describe my journey from my early days as a math major to my current career as a practicing computer scientist, mathematician, and board-certified ophthalmologist specializing in retina. The talk may be viewed here.

## March 31, Joshua Zelinsky, Birmingham-Southern College

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Title: Lower and Upper Bounds in Integer Complexity.

Abstract: Define ∥n∥ to be the complexity of n, the smallest number of 1’s needed to write n using an arbitrary combination of addition and multiplication. John Selfridge showed that ∥n∥≥ 3 log

_{3}n for all n, and this lower bound is obtained exactly when n is a power of 3. Richard Guy noted the trivial upper bound that ∥n∥≤ 3 log

_{2}n for all n > 1 by writing n in base 2. This talk will discuss work improving the upper bound as well as work leading to a complete classification of numbers whose complexity is close to the lower bound. Along the way, we’ll develop connections to both ordinal numbers as well as the p-adics.

## March 10, Yangyang Xu, University of Alabama

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Title: primal-dual methods for affinely constrained problems.

Abstract: Recent years have witnessed the popularity of first-order methods (that do not use second or higher-order derivative information) for extremely large-scale problems. In general, original first-order methods (e.g., proximal gradient) can converge slowly, and thus many efforts have been made to accelerate various first-order methods.

I will present a randomized primal-dual (RPD) method for solving multi-block structured problems. Without strong assumptions, multi-block ADMM can diverge. I will show that the randomization technique helps solve the divergence issue, and the RPD method converges in the rate of O(1/k) to a solution with merely convexity condition. The rate can be accelerated to O(1/k^2) if strong convexity on the objective is assumed. Finally, I will show a few numerical results to demonstrate the nice performance of the presented algorithms.

## March 3, Lex Oversteegen, UAB

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Title: Extensions of isotopies in the plane.

Abstract: Let A be any planar set. It is known that a holomorphic motion h : A × D → ℂ always extends to a holomorphic motion of the entire plane. It was recently shown that any isotopy h : X × [0,1] → ℂ, starting at the identity, of a planar continuum X also extends to an isotopy of the entire plane. Easy examples show that this result does not generalize to all planar compacta. In this talk we will provide a characterization of isotopies of planar uniformly perfect compacta X which extend to an isotopy of the entire plane. It will follow that such an extension is always possible provided the diameters of all components of X are uniformly bounded away from zero.

Joint work with: L. Hoehn and E. Tymchatyn

## February 24, Michael Yampolsky, University of Toronto

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Title: New results on renormalization and rigidity of analytic critical circle maps.

Abstract: I will give an introduction to renormalization and rigidity theory of analytic homeomorphisms of the circle with critical points and will present some recent results. No prior familiarity with the subject will be assumed.

## February 17, Logan Hoehn, Nipissing University

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Title: Homogeneity and dendroids

Abstract:

Given a topological space X, we consider the group H(X) of homeomorphisms of X to itself. There is a natural action of H(X) on X, defined simply by (h,x) \mapsto h(x). Topological symmetry, or homogeneity, of a space X can be quantified by considering the number of orbits in X under this action -- the fewer orbits there are, the more homogeneous X is.

A curious phenomenon is that many famous and beautiful topological spaces, particularly those with some type of universality property, exhibit a high degree of topological homogeneity. I will illustrate this phenomenon with several examples. I will also make a case for the converse: that spaces with a high degree of homogeneity tend to be rare and remarkable. I will focus on three classes of spaces: compact subsets of the plane R^2, dendrites, and dendroids.

## February 10, Jernej Činč, University of Vienna

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Title: Accessible points of chainable continua.

Abstract: We say that a continuum (compact, connected metric space) is chainable, if it can be covered by finite collections of open sets of an arbitrary small diameter so that despite itself only adjacent open sets from the cover intersect. A point x ∈ K ⊂ ℝ2 is accessible if there exists an arc A ⊂ ℝ2 such that A ∩ K = {x}. It is well known that chainable continua can be embedded in the plane. In this talk I will present a construction to obtain planar embeddings of a special class of chainable continua so that an arbitrary point from such continuum is accessible. Furthermore, I will discuss some results that can be derived from such construction. The talk will be based on the joint work with Ana Anušić (University of Zagreb, Croatia) and Henk Bruin (University of Vienna, Austria).

## February 3, Chris Mouron, Rhodes College

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Title: The topology of continua that admit fully expansive homeomorphisms.

Abstract: A continuum is a compact connected metric space. A homeomorphism h : X-→X is expansive if there exists a c > 0 such that for any distinct x,y ∈ X, there exists n ∈ ℤ such that d(h

^{n}(x),h

^{n}(y)) > c. A homeomorphism is positively continuum-wise fully expansive if for every pair ϵ,δ > 0 there is a N(ϵ,δ) > 0 such that if Y is a subcontinuum of X with diam(Y ) ≥ δ, then d

_{H}(h

^{n}(Y ),X) < ϵ for all n ≥ N(ϵ,δ), where d

_{H}is Hausdorff distance. That is, all proper subcontinua grow arbitraily close to X under the action of the homeomorphism. A homeomorphism is fully expansive if it is both expansive and positively continuum-wise fully expansive. In this talk I will give examples of fully expansive homeomorphisms and necessary conditions for a continuum to admit a fully expansive homeomorphism.

## January 27, Mareike Haberichter, Amherst

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- Category: 2016-2017

Title: Solitons: From Tsunamis to Skyrmions

Abstract: Topological solitons are stable, finite energy solutions to nonlinear field equations found in a variety of physical systems. Examples include vortices in superconductors, baby Skyrmions in condensed matter systems and the infamous Skyrmions of nuclear physics. In this talk, I will provide an introduction to topological solitons, with a particular focus on the solitons of the sine-Gordon and Skyrme models. We will also discuss some of the nuclear physics applications of the Skyrme model.

## December 2, Leonid Bunimovich, Georgia Tech

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- Category: 2016-2017

Title: Isospectral Transformations of Networks

Abstract: It is tempting, when dealing with multidimensional systems and networks, to reduce a system while keeping some its inportant characteristics. I will describe some basics of a new theory/approach which allows to keep allthe information on spectrum and eigenvectors of the system. So far all applications of this approach to various new and long standing problems were successful. I will also discuss some of them.

## November 18, Xaq Pitkow, Rice University

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Title: How the brain might work: statistics flowing through redundant population codes

Abstract: In this talk I will discuss a theory of computation by the brain, invoking four fundamental principles: The brain is nonlinear, recurrent, weighs uncertainty, and distributes information across multiple neurons. A brain model accounting for these principles performs approximate probabilistic inference based on a statistical model of the world. It is based on probabilistic graphical models, which are mathematical objects living at the intersection of graph theory and probability theory. Inference in such a model can be described by statistics flowing around the graph according to a message-passing algorithm. These low-dimensional messages are embedded in the high-dimensional nonlinear dynamics of the neural activity patterns. The actual neural transformations that implement this algorithm are degenerate, so I describe mathematical concepts to define the family of equivalent algorithms. Finally, I provide a data analysis framework needed to test this theory using neural activity patterns recorded in animal brains while the animal performs complex naturalistic tasks.

## November 11, Federico Bonetto, Georgia Tech

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Title: Thermostats and Heat Reservoirs for a Kac type model of gas.

Abstract: This work is part of a joint effort with Michael Loss to use

the Kac model to gain some understanding on some basic concepts in

Nonequilibrium Statistical Mechanics.

I will introduce the Kac model and present some classical results on its

approach to equilibrium. I will then couple it with a thermostat (an

infinite heat reservoir) and see how the previous results are affected.

I will show how the thermostat can be effectively modeled via a finite

heat reservoir and what this tells us on the general question of

approach to equilibrium.

## November 4, Ed Tymchatyn, University of Saskatchewan

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Title: Cell Structures

Abstract: Alexsandroff in 1926 began approximating compact metric spaces by polyhedra in order to establish the foundations of algebraic topology for metric compacta and more general spaces. Compact metric spaces are obtained as inverse limits of inverse sequences of finite polyhedra. A sequence of mappings between factor spaces which commute with bonding mappings of two such inverse spectra defines a mapping between the the inverse limit spaces. Unfortunately the converse is not true in general. It took more than 60 years to fully develop Alexsandroff's idea. The resulting machinery is daunting.

Twenty five years ago Debski realized that the trouble with Alexsandroff's approach was that it requires too much information to be encoded in the inverse systems.

Debski-T in a paper to appear in Coll. Math. define cell structures as inverse sequences of graphs (i.e. discrete sets with a reflexive and symmetric relation) with mild convergence conditions. They also define cell maps between cell structures. In that paper they show that these suffice to define all complete metric spaces and mappings between such spaces.

In this talk I will show by how to define the real line and mappings of the line to itself using such discrete approximations. I will then explain how to extend the ideas in that example to obtain all topologically complete spaces and continuous maps between such spaces.

Conclusion: Most reasonable spaces and continuous mappings are determined by families of discrete approximations.

## October 28, Siyuan Lu, McGill University

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Title: On Weyl's embedding problem in Riemannian manifold

Abstract: Weyl's embedding problem is a classic isometric embedding problem in differential geometry. It concerns how to isometrically embed (S^2,g) with positive Gauss curvature into R^3. Recent study of quasi-local mass in General Relativity motivates us to study Weyl's embedding problem in general Riemannian manifold. In this talk, we focus on the mean curvature estimate for the embedding, which is the crucial part in view of continuity method. Together with work by Li-Wang, we are able to obtain an isometric embedding in certain Riemannian manifold. Moreover, we are able to recover the classic Weyl's embedding theorem in space form under the condition that g\in C^2 with D^2g Dini continuous, which is new even in Euclidean space.

## October 21, Kevin Pilgrim, Indiana University Bloomington

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Title: Semigroups of branched mapping classes

Recent work on classification problems arising in one-dimensional complex analytic dynamics suggest an underlying theory of mapping class semigroups. Let S2 denote the two-sphere, and fix a finite set P ⊂ S2. The set BrCov(S2, P) of orientation-preserving branched covering maps of pairs f: (S2, P) → (S2, P) of degree at least two and whose branch values lie in P is closed under composition and under pre- and post-composition by orientation-preserving homeomorphisms h: (S2, P) → (S2, P) fixing P set wise. Composition descends to a well-defined map on homotopy classes relative to P, yielding a countable semigroup BrMod(S2, P). In addition to the semigroup structure, BrMod(S2, P) is naturally equipped with two commuting actions of the mapping class group Mod(S2, P), induced by pre- and post-composition. This richer biset structure, and a related circle of constructions, turn out to be extraordinarily useful in this context. They lead to: algebraic invariants of elements of BrCov(S2, P) and an analog of the Baer-Dehn-Nielsen theorem; analogs of classical Hurwitz classes; conjugacy invariants; canonical decompositions and forms; an analog of Thurston's trichotomous classification of mapping classes; and induced dynamics on Teichmüller spaces.

This talk is based on algebraic and dynamical perspectives growing out of work of L. Bartholdi and V. Nekrashevych, and is based on ongoing conversations with S. Koch, D. Margalit, and N. Selinger.

## October 14, Jason Teutsch, UAB

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Title: A brief on short descriptions

Abstract: Given a binary string, can one find a short description for it? The well-known answer is "no, Kolmogorov complexity is not computable.'' Faced with this barrier, one might instead seek a short list of candidates which includes a laconic description. In fact, efficiently computable short lists do exist, and I will discuss the extent to which one can obtain them. This talk will include a gentle introduction to Kolmogorov complexity followed by a discussion connecting list approximations to classical combinatorics and randomness extraction. The program will roughly follow a recent SIGACT News survey coauthored with Marius Zimand.

## October 7, Hyun-Kyoung Kwon, University of Alabama

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Title: The Corona Theorem and its Applications

Abstract: The original proof of the Corona Theorem is due to L. Carleson and many different versions and generalizations of the Corona Theorem have been investigated. I will discuss one such generalization recently obtained with my graduate student. I will also explain how the technique widely used to solve the Corona problems can be applied to get results about operator equivalence (unitary equivalence and similarity) and about the degree bound in Hilbert's Nullstellensatz.

## September 30, Kabe Moen, University of Alabama

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Abstract: We survey some basic facts about Lebesgue spaces. We show that the union of Lebesgue spaces is intimately related to the Hardy-Littlewood maximal function and the theory of weighted Lebesgue spaces -- Lebesgue spaces with a change of measure. We give several simple characterizations of when a function belongs the union Lebesgue spaces. This presentation will be based on a joint work with Greg Knese and John McCarthy.

## September 23, Jesse Prince-Lubawy, University of North Alabama

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Abstract: A handlebody orbifold consists of finitely many quotients of the 3-ball by spherical groups (

**,**

*Z*_{n}**,**

*D*_{n}**,**

*A*_{4}**, and**

*S*_{4}**) connected by 1-handle orbifolds respecting singular axes and their orders, and such that topologically the outcome is an orientable handlebody. We will first examine the handlebody of genus two,**

*A*_{5}*V*, and the handlebody orbifolds

_{2}*V*/

_{2}*G*, where

*G*is a finite group. We will then discuss equivalence of group actions and see that, up to equivalence, there are 13 actions on

*V*. This will lead into my work, where we consider cyclic

_{2}*p*-squared actions, where

*p*is prime, on a handlebody of genus

*g*.

## September 16, Ted Mahavier, Lamar University

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Abstract: Solving differential equations using steepest descent methods based on the Euclidean norm has long been established as ineffective, although pre-conditioning techniques may alleviate this problem to some extent. However, steepest descent in spaces with a better choice of norm can be quite efficient. Beginning with an example accessible to undergraduates, we will outline Sobolev descent on a few elementary examples and demonstrate at least one interesting open problem in the area. This talk should be accessible to undergraduates, graduate students and faculty not necessarily experts in numerical differential equations.