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Mathematics & Statistics
Colloquium Archive

# Colloquia — Spring 2015

## Friday, May 1, 2015

Title
Speaker

Time
Place

Using lattices for computational number theory
Jean-François Biasse
University of Waterloo
Waterloo, ON
3:00pm-4:00pm
CMC 130
Lesɫaw Skrzypek

Abstract

We begin this talk with a computational problem attributed to Archimedes. It can be solved by computing the unit group of a number field, which is a fundamental task in number theory.

Then I will introduce the state-of-the-art methods for computing the ideal class group and the unit group of a number field. We will see how these methods allow us to check if an ideal in the ring of integers of a number field is principal, and if so, compute a generator. This fundamental task has recently received a lot of attention from the cryptographic community because a series of lattice-based schemes rely on its difficulty.

Finally, I will mention the state of the art methods for solving these problems with quantum computers. These algorithms are typical examples of cases where quantum computers can drastically improve on the theoretical complexity. We will conclude by mentioning the relevance of these results to what is called “post-quantum cryptography”.

## Friday, April 10, 2015

Title
Speaker

Time
Place

The index approach to permutation polynomials over finite fields
Steve Wang
Carleton University
3:00pm-4:00pm
CMC 130
Xiang-dong Hou

Abstract

A permutation polynomial (PP) $$f$$ over a finite field is a nonzero polynomial that acts as a permutation of the elements of the field, i.e., the map $$x\rightarrow f(x)$$ is one-to-one. Equivalently, the size of the value set of $$f$$ is the extreme case. The study of permutation polynomials over a finite field goes back to 19th century when Hermite and later Dickson pioneered this area of research. In recent years, permutation polynomials have attracted a lot of attention due to their applications in many areas of science and engineering. As a consequence, many interesting discoveries have been made recently by various researchers.

However, the classification of PPs is still out of reach. Most current research concentrate on constructions, distribution, and enumeration of PPs in terms of parameters of finite fields and parameters of polynomials such as coefficients and exponents (including the degree). In this talk I will report some general results on PPs, in particular, an index approach to study PPs over finite fields. We note that index is another parameter for any polynomial over a finite field, which was newly introduced by Akbary, Ghioca and the speaker. This parameter turns out to be also very useful in studying several problems on polynomials over finite fields.

## Friday, April 3, 2015

Title
Speaker

Time
Place

From exponential bases to the discrete Hilbert transform
Laura De Carli
Florida International University
3:00pm-4:00pm
CMC 130
Dmitry Khavinson

Abstract

The discrete Hilbert transform was first studied by D. Hilbert and H. Weil in 1908 and has generated interest among the mathematician ever since.

In this talk I will show how a seemingly simple problem on exponential bases on $$L^2$$ lead to the investigation of a one-parameter semigroup of operators on $$l^2$$ whose infinitesimal generator is the discrete Hilbert transform. If time allows, I will also present other families of discrete operator that appear in connection with problems on exponential bases. Part of this work-in-progress is joint with my student Shaikh Gohin Samar, who is completing his Master's at FIU. The talk will be accessible to graduate and advanced undergraduate students in Mathematics

## Friday, March 27, 2015

Title
Speaker

Time
Place

Determinantal representations of polynomials
Greg Knese
Washington University
3:00pm-4:00pm
CMC 130
Dima Khavinson

Abstract

When can a polynomial be written as a determinant of a linear combination of matrices? Why would we want to do such a thing? These questions get really interesting when we restrict the polynomial or restrict the matrices under consideration. For example, one can consider real polynomials with no zeros on a product of upper half planes (a generalization of a real-zero polynomial in one variable) and representations using self-adjoint matrices. Such polynomials occur naturally in analysis, combinatorics, and optimization. We will discuss some of these connections as well as other variations on the polynomials and matrices we consider.

## Wednesday, March 25, 2015

Title
Speaker

Time
Place

Groups Arising at the Transition to Asphericity
William Bogley
Oregon State University
3:00pm-4:00pm
CMC 109
Dmytro Savchuk

Abstract

The homotopy type of an aspherical simplicial or CW complex is uniquely determined by its fundamental group, so homotopy invariants of an aspherical space are invariants of its fundamental group. I will describe asphericity as it relates to relative group presentations and present applications to the theory of cyclically presented groups. Through work of several authors over the past two decades, details are emerging of an interface where aspherical relative presentations transition to non-aspherical ones. Focusing on this interface, I will describe joint work with Gerald Williams in which we discover infinite families of efficient finite groups that admit short presentations and whose orders involve all Mersenne numbers, as well as other conjecturally infinite families of rational primes.

## Friday, March 20, 2015

Title
Speaker

Time
Place

Stochastic discrete two-dimensional growth
Mark Mineev-Weinstein
New Mexico Consortium
US & International Institute of Physics, Brazil
4:00pm-5:00pm
CMC 130
Razvan Teodorescu

Abstract

We consider a stochastic discrete growth process in two dimensions, in which randomly-distributed sources issue an arbitrary number of diffusing particles that aggregate upon contact with clusters. We calculate the probability of obtaining various shapes for clusters with the same area. After performing functional integration in a quasi-classical limit (when the number of aggregated particles goes to infinity, while the area deposited by a single random walker goes to zero), we obtain that the probability to obtain a given domain is given by the tau-function of the dispersionless integrable Today hierarchy, minus the term corresponding to the Coulomb self-energy of the cluster.

We will conclude with a new approach to pattern selection in light of this specific model, relate it to the entropy production, and discuss the derivation of classical Laplacian growth as the extremal of a generalized stochastic functional.

Title
Speaker

Time
Place

Finite Type Enhancements
Sam Nelson
Claremont McKenna College, CA
3:00pm-4:00pm
CMC 130

Abstract

Finite type invariants, also known as Vassiliev invariants, are a type of integer-valued knot and link invariant. In this talk we will see a method for using finite type invariants of biquandle-colored knots to enhance the quandle counting invariant.

## Monday, March 16, 2015

Title
Speaker

Time
Place

A geometric approach to understanding neural codes in recurrent networks
Carina Curto
Pennsylvania State University
2:00pm-3:00pm
CMC 108
Nataša Jonoska

Abstract

Synapses in many cortical areas of the brain are dominated by local, recurrent connections. It has long been suggested, therefore, that cortical networks may serve to restore a noisy or incomplete signal by evolving it towards a stored pattern of activity. These “preferred” activity patterns are constrained by the network's connections, and are typically modeled as stable fixed points of the dynamics. In this talk I will briefly review the permitted and forbidden sets model for cortical networks, first introduced by Hahnloser et al. (Nature, 2000), and then present some recent results that provide a geometric handle on permitted sets. Specifically, I will show how questions about fixed points can be translated to questions in classical distance geometry. Finally, I will use the geometric description of fixed points to show that these networks can perform error correction and pattern completion for a wide range of connectivities.

## Friday, March 13, 2015

Title
Speaker

Time
Place

Categorification in Applied Mathematics
Robert Ghrist
University of Pennsylvania
1:30pm-2:30pm
CMC 130
Dmytro Savchuk

Abstract

One of the organizing principles in Mathematics is that of categorification — the systematic lifting of numerical equalities to isomorphisms of higher algebraic objects: e.g., much of algebraic topology consists of categorification of numerical counts. This talk will be a gentle survey of several ways in which categorification lurks in applied mathematics, with classical and modern ideas alike having enrichments to algebraic structures that reveal richer relationships than numerical equality.

Title
Speaker

Time
Place

A topological approach for investigating the intrinsic structure of neural activity
Pennsylvania State University
3:00pm-4:00pm
CMC 130
Nataša Jonoska

Abstract

Experimental neuroscience is achieving rapid progress in the ability to collect neural activity and connectivity data. This holds promise to directly test many theoretical ideas, and thus advance our understanding of “how the brain works”. Detecting meaningful structure in this data is challenging because of unknown nonlinearities, where measured quantities are related to more “fundamental” variables by an unknown nonlinear transformation. We find that combinatorial topology can be used to obtain meaningful answers to questions about the structure of neural activity and introduce an approach that extracts features of the data invariant under arbitrary nonlinear monotone transformations. These features can be used to distinguish random and geometric structure, and depend only on the relative ordering of matrix entries. We apply our technique to neural activity in rat hippocampus, and find that the intrinsic pattern of correlations possesses a geometric organization in both spatial and non-spatial behaviors.

## Friday, February 27, 2015

Title
Speaker

Time
Place

Orthogonal polynomials for normal matrix models with discrete rotational symmetries
Ferenc Balogh
Concordia University
4:00pm-5:00pm
CMC 130
Razvan Teodorescu

Abstract

Statistical observables of random unitary invariant normal matrix models can be expressed in terms of the joint probability distribution of the matrix eigenvalues. This leads to a logarithmic Coulomb gas model in which the eigenvalues are thought of as charged particles in the complex plane under the influence of an external potential.

The joint density of the eigenvalues can be written as a determinant with entries assembled from planar orthogonal polynomials associated to the given background potential. As the matrix size goes to infinity, the asymptotics of eigenvalue statistics, up to leading term, are encoded into the equilibrium measure, the solution of the continuum limit of the Coulomb gas variational problem. To obtain more refined scaling limits, one needs to study the strong asymptotics of the corresponding orthogonal polynomials. After a brief review of the known results to date, I will introduce a special one-parameter normal matrix model with a discrete rotational symmetry for which the equilibrium measure can be found explicitly for all values of the parameter, including a critical value where a non-trivial topological transition of the support is observed. It will be shown how the corresponding orthogonal polynomials can be analyzed using nonlinear steepest descent techniques, based on a trick of writing two-dimensional orthogonality relations in terms of contour integrals, leading to a Riemann-Hilbert problem. In particular, The results confirm a conjectured relation between the limiting zero distribution of the orthogonal polynomials and the equilibrium measure via a balayage procedure.

The talk is based on joint works with T. Grava and D. Merzi.

Title

Speaker

Time
Place

Why the oracle may not exist: ergodic families of Jacobi matrices, absolute continuity without almost periodicity
Alexander Volberg
Michigan State University
3:00pm-4:00pm
CMC 130
Arthur Danielyan

Abstract

We will explain the recent solution of Kotani's problem pertinent to the existence/non-existence of “oracle” (almost periodicity) for the ergodic families of Jacobi matrices (discrete Schröedinger operators). Kotani suggested that such families are subject to the following implication: if family has a non-trivial absolutely continuous spectrum (this happens almost surely) then almost surely it consists of almost periodic matrices (hence the possibility to predict the future by the past). Kotani proved an important positive result of this sort. Recently independently Artur Avila and Peter Yuditskii—myself disproved this conjecture of Kotani (by two different approaches). We will show the hidden singularity that defines when such Kotani's oracle exists or not.

## Tuesday, February 24, 2015

Title
Speaker

Time
Place

Conformal mapping and random Laplacian growth
Alan Sola
University of Cambridge
3:35pm-4:45pm
CMC 109
Wen-Xiu Ma

Abstract

In my talk, I will survey recent advances in the conformal mapping approach to Laplacian random growth in the plane, where aggregating particles are represented by simple conformal maps, and growth of aggregates is encoded through composition of random copies of such maps.

The random planar sets one obtains in this way exhibit rich and fascinating structures, but the analysis of the large-scale geometry and microscopic features of these so-called clusters present a formidable challenge to mathematicians, with many basic questions remaining wide open.

Computer simulations will be used to illustrate the results obtained, and also to formulate a number of open problems.

## Friday, February 20, 2015

Title

Speaker

Time
Place

Asymptotic regimes of complex orthogonal polynomials with varying quartic weight: Global behaviour critical points
Alexander Tovbis
University of Central Florida
3:00pm-4:00pm
CMC 130
Seung-Yeop Lee

Abstract

We study the asymptotics of recurrence coefficients for monic orthogonal polynomials $$\pi_n(z)$$ with the quartic exponential weight $$\exp\left[-N\left(\frac{z^2}2+\frac{tz^4}4\right)\right]$$, where $$t\in\mathbb{C}$$ and $$N\in\mathbb{N}$$, $$N\to\infty$$. Our goal is to describe the regions of different asymptotic behaviour globally in $$t\in\mathbb{C}$$ as well as behaviour near the critical points.

Title
Speaker

Time
Place

The Homotopy Analysis Method for Differential Equations, and Hierarchies of Integrable Systems
Mathew Baxter
University of Central Florida
2:00pm-3:00pm
CMC 130
Wen-Xiu Ma

Abstract

The Homotopy Analysis Method is an innovative new way (Liao, 1992) to get analytical solutions to nonlinear di erential equations. We begin with a brief introduction to the concept of homotopy from topology. From there, the Homotopy Analysis Method is discussed in detail. We describe how the idea of homotopy is applied to introduce a parameter into ordinary/partial differential equations that do not have one to begin with. The homotopy between a linear operator and a nonlinear operator allows us to use perturbation on this parameter to obtain analytical solutions to these equations with small error. The idea is applied to an equation governing the nonlinear evolution of a vector potential of an electromagnetic pulse propagating in an arbitrary pair plasma with temperature asymmetry. We also look at the Hasegawa-Mima equation, a very difficult PDE that governs the electric potential due to a drift wave in a plasma. Future work is discussed.

After this, hierarchies of integrable systems will be discussed. In particular we look at the integrability of the Zakharov-Ito hierarchy due to its zero-curvature representation. We find the bi-Hamiltonian structure, and show the Hamiltonians are involute in pairs under a well-defined Poisson bracket, implying that the equations in our hierarchy are Liouville integrable. Future work is discussed.

## Tuesday, February 17, 2015

Title
Speaker

Time
Place

Augmented eigenfunctions: a new spectral object appearing in the integral representation of the solution of linear initial-boundary value problems
David Smith
University of Cincinnati
3:45pm-4:45pm
CMC 109
Wen-Xiu Ma

Abstract

We study initial-boundary value problems for linear, constant-coefficient partial differential equations of arbitrary order, on a finite or semi-infinite domain, with arbitrary boundary conditions. It has been shown that the recent Unified Transform Method of Fokas can be used to solve all such classically well-posed problems. The solution thus obtained is expressed as an integral, which represents a new kind of spectral transform. We compare the new method, and its solution representation, with classical Fourier transform techniques, and their resulting solution representations. In doing so, we discover a new species of spectral object, encoded by the spectral transforms of the Unified Method.

## Friday, February 13, 2015

Title
Speaker

Time
Place

Quantum invariants of knots, hyperbolic geometry and $$q$$-series
Mustafa Hajij
Louisiana State University
4:00pm-5:00pm
CMC 130
Wen-Xiu Ma

Abstract

The discovery of the Jones polynomial lead to a vast family of invariants called the quantum invariants. Quantum invariants deeply connect many domains of mathematics such as quantum groups, hyperbolic geometry, knot theory and number theory. In this talk I will talk about quantum invariants and some of their connections with the geometry of the knot complement. Furthermore, I will describe some recent connections with number theory.

Title
Speaker

Time
Place

Random Matrices and Potential Theory
Thomas Bloom
University of Toronto
3:00pm-4:00pm
CMC 130
Vilmos Totik

Abstract

I will introduce the Gaussian Unitary Ensemble and other unitary ensembles of random matrices.

I will discuss aspects of these ensembles which can be studied and established via potential theory.

Specifically,I will deal with the convergence of the empirical measure of the eigenvalues and large deviation principles.

## Tuesday, February 10, 2015

Title
Speaker

Time
Place

Algebraic Geometry and Approximation Theory
Michael DiPasquale
University of Illinois at Urbana-Champaign
4:30pm-5:30pm
CHE 302
Wen-Xiu Ma

Abstract

Piecewise polynomial functions, also known as splines, are a cornerstone of approximation theory today. A question of fundamental interest in spline theory is to determine the dimension of (and a basis for) the vector space of splines of degree at most $$\mathbf{d}$$ over a polytopal complex. We give a survey of some ways in which this question may be approached from the perspective of commutative algebra and algebraic geometry. Key players in this story are the Hilbert function, the Hilbert polynomial, and Castelnuovo-Mumford regularity.

Title
Speaker

Time
Place

$$q$$-polynomial invariant of rooted trees; state sum invariant of graphs
Jozef Przytycki
George Washington University
3:30pm-4:30pm
ISA 3050

Abstract

We describe in this note a new invariant of rooted trees and following up state sum invariant of pointed graphs.

We argue that the invariant is interesting on it own, and that it has connections to knot theory and homological algebra. Another reason that we propose this invariant is that we deal here with an elementary, interesting an new mathematics and after the Colloquium everybody can take part in developing the topic inventing new results and connections to other disciplines of mathematics (and likely statistical mechanics and combinatorial biology).

## Friday, February 6, 2015

Title
Speaker

Time
Place

Pattern formation and bifurcations in reaction-diffusion-advection ecological models
Junping Shi
College of William and Mary
Williamsburg, VA
3:00pm-4:00pm
CMC 130
Yuncheng You

Abstract

Spatial-temporal patterns appear often in historical ecosystem data, and the cause of the patterns can be attributed to various internal or external forces. We demonstrate that in spatial ecological models, spatial-temporal patterns can arise as a result of self-organization of the ecosystem. By using bifurcation theory, we show that the spatial-temporal patterns are generated with the effect of diffusion, advection, chemotaxis or time delay.

Title
Speaker

Time
Place

Planar functions and their importance in cryptography
Stephen M. Gagola
University of Witwatersrand
2:00pm-3:00pm
CMC 130
Brian Curtin

Abstract

Planar functions were first introduced by Dembowski and Ostrom. Since 1991 such functions have attracted interest in cryptography as fuctions with optimal resistance to differential cryptanalysis. They were first used in this way by Nyberg where they were given another name “perfect nonlinear” which describes their important cryptographic property of being as far from linear as possible.

Now planar functions have applications in classical cryptographic systems, quantum cryptographic systems, wireless communication, and coding theory. Commutative semifields are equivalent to those planar functions that are known as Dembowski-Ostrom polynomials (DO polynomials). Here I will introduce how Joanne Hall and I have developed methods of constructing families of planar functions and commutative semifields of order $$p^{2r}$$ for any odd prime $$p$$ and any positive integer $$r$$. These families yield a more general construction which includes some other families of known planar functions while at the same time creates new classes of planar functions. Subsequently these were used to construct mutually unbiased bases, a structure of importance in quantum information theory.

## Friday, January 30, 2015

Title
Speaker

Time
Place

Combining Riesz bases
Shahaf Nitzan
Kent State University
4:00pm-5:00pm
CMC 130
Catherine Bénéteau

Abstract

Orthonormal bases (ONB) are used throughout mathematics and its applications. However, in many settings such bases are not easy to come by. For example, it is known that even the union of as few as two intervals may not admit an ONB of exponentials. In cases where there is no ONB, the next best option is a Riesz basis (i.e. the image of an ONB under a bounded invertible operator).

In this talk I will discuss the following question: Does every finite union of rectangles in $$R^d$$, with edges parallel to the axes, admit a Riesz basis of exponentials? In particular, does every finite union of intervals in $$R$$ admit such a basis?

This is joint work with Gady Kozma.

Title
Speaker

Time
Place

Mathematics in Cryptography: Today's Applications and Tomorrow's Foundations
Rainer Steinwandt
Florida Atlantic University
3:00pm-4:00pm
CMC 130
Brian Curtin

Abstract

The first part of the talk will show how mathematical techniques are applied in the design of modern cryptographic protocols. Taking the task of establishing a secret key among $$n\ge2$$ users over an insecure network as example, we discuss how computational assumptions enable the derivation of an efficient solution with strong provable guarantees.

Regrettably, some of the most common assumptions needed for today's cryptographic solutions are no longer justifiable in a so-called post-quantum scenario. In particular, popular constructions involving elliptic curves are not available in this setting. Post-quantum cryptography is of interest when cryptographic solutions are expected to guarantee security for many years. The cryptographic community is currently trying to identify mathematical platforms for efficient post-quantum solutions of basic cryptographic tasks like public-key encryption or digital signatures. The second part of the talk will discuss some of the current approaches, including in particular attempts that invoke tools from group theory.

## Friday, January 23, 2015

Title
Speaker

Time
Place

Self-similar groups and expanding graphs
Ievgen Bondarenko
Kyiv National University
3:00pm-4:00pm
CMC 130
Dmytro Savchuk

Abstract

Expanding graphs are highly connected sparse graphs which have numerous applications in mathematics and theoretical computer science. Reingold, Vadhan, and Wigderson (2002) discovered a simple combinatorial construction of expanding graphs. This construction was based on the new operation on regular graphs — the zig-zag product, which is closely related to the replacement product of graphs.

In this talk I will describe how to model iterated replacement product and zig-zag product of graphs by finite automata. Also I will explain a simple construction of self-similar groups whose action graphs produce a family of expanding graphs.

Title
Speaker

Time
Place