Seminars and Colloquia by Series

Bridge trisections of knotted surfaces in the four-sphere

Series
Geometry Topology Seminar
Time
Thursday, December 3, 2015 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Jeff Meier – University of Indiana

Please Note: Please not non-standard day for seminar.

A trisection is a decomposition of a four-manifold into three trivial pieces and serves as a four-dimensional analogue to a Heegaard decomposition of a three-manifold. In this talk, I will discuss an adaptation of the theory of trisections to the relative setting of knotted surfaces in the four-sphere that serves as a four-dimensional analogue to bridge splittings of classical knots and links. I'll show that every such surface admits a decomposition into three standard pieces called a bridge trisection. I'll also describe how every such decomposition can be represented diagrammatically as a triple of trivial tangles and give a calculus of moves for passing between diagrams of a fixed surface. This is joint work with Alexander Zupan.​

Orthogonal polynomials for the Minkowski question Mark function

Series
Analysis Seminar
Time
Wednesday, December 2, 2015 - 14:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Walter Van Assche – University of Leuven, Belgium
The Minkowski question mark function is a singular distribution function arising from Number Theory: it maps all quadratic irrationals to rational numbers and rational numbers to dyadic numbers. It generates a singular measure on [0,1]. We are interested in the behavior of the norms and recurrence coefficients of the orthonormal polynomials for this singular measure. Is the Minkowski measure a "regular" measure (in the sense of Ullman, Totik and Stahl), i.e., is the asymptotic zero distribution the equilibrium measure on [0,1] and do the n-th roots of the norm converge to the capacity (which is 1/4)? Do the recurrence coefficients converge (are the orthogonal polynomials in Nevai's class). We provide some numerical results which give some indication but which are not conclusive.

The Cauchy problem for the pressureless Euler/isentropic Navier-Stokes equations

Series
PDE Seminar
Time
Tuesday, December 1, 2015 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Young-Pil Choi – Imperial College London
The interactions between particles and fluid have received a bulk of attention due to a number of their applications in the field of, for example, biotechnology, medicine, and in the study of sedimentation phenomenon, compressibility of droplets of the spray, cooling tower plumes, and diesel engines, etc. In this talk, we present coupled hydrodynamic equations which can formally be derived from Vlasov-Boltzmann/Navier-Stokes equations. More precisely, our proposed equations consist of the compressible pressureless Euler equations and the isentropic compressible Navier-Stokes equations. For the coupled system, we establish the global existence of classical solutions when the domain is periodic, and its large-time behavior which shows the exponential alignment between two fluid velocities. We also remark on blow-up of classical solutions in the whole space.

Joseph Ford Commemorative Colloquium - Synchronization in Populations of Chemical Oscillators - Quorum Sensing, Phase Clusters and Chimera

Series
Other Talks
Time
Monday, November 30, 2015 - 15:05 for 1 hour (actually 50 minutes)
Location
Howey L3
Speaker
Kenneth Showalter – West Virginia University

Please Note: Hosted by Roman Grigoriev, School of Physics

We have studied large, heterogeneous populations of discrete chemical oscillators (~100,000) to characterize two different types of density-dependent transitions to synchronized behavior, a gradual Kuramoto synchronization and a sudden quorum sensing synchronization. We also describe the formation of phase clusters, where each cluster has the same frequency but is phase shifted with respect to other clusters, giving rise to a global signal that is more complex than that of the individual oscillators. Finally, we describe experimental and modeling studies of chimera states and their relation to other synchronization states in populations of coupled chemical oscillators.

Extremal Matrices for Graphs without K_5 Minors

Series
Algebra Seminar
Time
Monday, November 30, 2015 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Liam Solus – University of Kentucky –
Given a graph G on p vertices we consider the cone of concentration matrices associated to G; that is, the cone of all (p x p) positive semidefinite matrices with zeros in entries corresponding to the nonedges of G. Due to its applications in PSD-completion problems and maximum-likelihood estimation, the geometry of this cone is of general interest. A natural pursuit in this geometric investigation is to characterize the possible ranks of the extremal rays of this cone. We will investigate this problem combinatorially using the cut polytope of G and its semidefinite relaxation, known as the elliptope of G. For the graphs without K_5 minors, we will see that the facet-normals of the cut polytope identify a distinguished set of extremal rays for which we can recover the ranks. In the case that these graphs are also series-parallel we will see that all extremal ranks are given in this fashion. Time permitting, we will investigate the potential for generalizing these results. This talk is based on joint work with Caroline Uhler and Ruriko Yoshida.

Modeling and Controllability issues for a general class of smart structures, a general outlook

Series
Applied and Computational Mathematics Seminar
Time
Monday, November 30, 2015 - 11:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Dr. Ahmet Özkan Özer – University of Nevada-Reno –
In many applications, such as vibration of smart structures (piezoelectric, magnetorestrive, etc.), the physical quantity of interest depends both on the space an time. These systems are mostly modeled by partial differential equations (PDE), and the solutions of these systems evolve on infinite dimensional function spaces. For this reason, these systems are called infinite dimensional systems. Finding active controllers in order to influence the dynamics of these systems generate highly involved problems. The control theory for PDE governing the dynamics of smart structures is a mathematical description of such situations. Accurately modeling these structures play an important role to understanding not only the overall dynamics but the controllability and stabilizability issues. In the first part of the talk, the differences between the finite and infinite dimensional control theories are addressed. The major challenges tagged along in controlling coupled PDE are pointed out. The connection between the observability and controllability concepts for PDE are introduced by the duality argument (Hilbert's Uniqueness Method). Once this connection is established, the PDE models corresponding to the simple piezoelectric material structures are analyzed in the same context. Some modeling issues will be addressed. Major results are presented, and open problems are discussed. In the second part of the talk, a problem of actively constarined layer (ACL) structures is considered. Some of the major results are presesented. Open problems in this context are discussed. Some of this research presented in this talk are joint works with Prof. Scott Hansen (ISU) and Kirsten Morris (UW).

The Erdos-Hajnal Conjecture and structured non-linear graph-based hashing

Series
Graph Theory Seminar
Time
Monday, November 23, 2015 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Krzysztof Choromanski – Google Research
The goal of this talk is to show recent advances regarding two important mathematical problems. The first one can be straightforwardly formulated in a graph theory language, but can be possibly applied in other fields. The second one was motivated by machine learning applications, but leads to graph theory techniques. The celebrated open conjecture of Erdos and Hajnal from 1989 states that families of graphs not having some given graph H as an induced subgraph contain polynomial-size cliques/stable sets (in the undirected setting) or transitive subsets (in the directed setting). Recent techniques developed over last few years provided the proof of the conjecture for new infinite classes of graphs (in particular the first infinite class of prime graphs). Furthermore, they gave tight asymptotics for the Erdos-Hajnal coefficients for many classes of prime tournaments as well as the proof of the conjecture for all but one tournament on at most six vertices and the proof of the weaker version of the conjecture for trees on at most six vertices. In this part of the talk I will summarize these recent achievements. Structured non-linear graph-based hashing is motivated by applications in neural networks, where matrices of linear projections are constrained to have a specific structured form. This drastically reduces the size of the model and speeds up computations. I will show how the properties of the underlying graph encoding correlations between entries of these matrices (such as its chromatic number) imply the quality of the entire non-linear hashing mechanism. Furthermore, I will explain how general structured matrices that very recently attracted researchers’ attention naturally lead to the underlying graph theory description.

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