The ISU Department of Mathematics Colloquium is organized by

Pablo Raúl Stinga (stinga@iastate.edu)

Upcoming Fall 2019

**November 5
**

**Ryan Martin
**

**: Graph Turan numbers and symmetrization methods**

Title

Title

**Abstract:**

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**Spring 2019**

**Tuesdays 4:10 p.m. in**

__Carver 0001__- Tea and cookies starting at 3:45 p.m. in Carver 404

**January 14
**

**Brian Collier
**

**January 15
**

**Ruoyu Wu
**

**January 17
**

**Ryan Goh
**

**January 22
**

**Carl Wang-Erickson
**

**January 23
**

**Rohit Nagpal
**

**January 24
**

**Radoslav Fulek
**

**January 28
**

**Noah Schweber
**

**February 1
**

**Gabriel Conant
**

Title: Arithmetic regularity with forbidden bipartite configurations

Title: Arithmetic regularity with forbidden bipartite configurations

**Abstract:**Szemeredi’s regularity lemma is a fundamental result in graph theory, which says that sufficiently large finite graphs can be partitioned into a small number of pieces so that the edges between most pairs of pieces are randomly distributed. In other words, the regularity lemma processes finite graphs into ingredients that are either highly structured (namely, the partition of the graph) or highly random (namely, the edges between regular pairs). In 2005, Green introduced arithmetic regularity, which is a group-theoretic analogue of Szemeredi’s result. Green’s work involves decompositions of finite abelian groups into algebraically structured pieces, which behave randomly with respect to some chosen subset of the group. In this talk, I will consider a set $A$ in an arbitrary finite group $G$, such that the bipartite graph on $G\times G$ induced by $x\cdot y\in A$ omits some induced bipartite subgraph of size bounded by a fixed integer $k$. I will present strengthened versions arithmetic regularity in this setting, which yield algebraic structure theorems for sets $A$ as above. This work combines tools from model theory, additive combinatorics, and the structure of compact topological groups. Joint with A. Pillay and C. Terry.

**February 4
**

**Sun Kim
**

**Title: Two identities in Ramanujan’s Lost
Notebook & Bressoud’s conjecture**

**Abstract:** In this talk, we discuss two
topics. The first topic is about two identities
that Ramanujan recorded without proofs in his lost
notebook. These two identities are intimately
connected with the classical Circle and Divisor
problems in number theory, respectively. They
involve doubly infinite series of Bessel
functions, and in each case, there are three
possible interpretations for the double series. We
proved the first identity under all three
interpretations, and the second under two of them.
Furthermore, we discuss many analogues and
generalizations of them. This is joint work with
Bruce C. Berndt and Alexandru Zaharescu.

The second topic is about Bressoud’s conjecture.
In 1980, Bressoud obtained an analytic
generalization of the Rogers-Ramanujan-Gordon
identities. He then tried to establish a
combinatorial interpretation of his identity,
which specializes to many well-known
Rogers-Ramanujan type identities. He proved that a
certain partition identity follows from his
identity in a very restrictive case and
conjectured that the partition identity holds true
in general. We discuss Bressoud’s conjecture for
the general case and bijective proofs of it.

**February 7
**

**Jack Jeffries
**

University of Michigan

**Title**: From Zariski-Nagata to
local fundamental groups

**Abstract:**Hilbert's Nullstellensatz gives a dictionary between algebra and geometry; e.g., solution sets to polynomial equations over the complex numbers (varieties) translate to (radical) ideals in polynomial rings. A classical theorem of Zariski-Nagata gives a deeper layer to this correspondence: polynomial functions that vanish to certain order along a variety correspond to a natural algebraic notion called symbolic powers.

This is based on joint projects with Holger Brenner, Alessandro De Stefani, Eloísa Grifo, Luis Núñez-Betancourt, and Ilya Smirnov.

**February 11
**

**Chen-Yun Lin
**

**Title: Data and Curse of Dimensionality;
Spectral Geometry comes to rescue!**

**Abstract:** High dimensional data is
increasingly available in many fields. However, the
analysis of such data suffers the so-called curse of
dimensionality. One powerful approach to nonlinear
dimensionality reduction is the diffusion-type maps.
Its continuous counterpart is the embedding of a
manifold using the eigenfunctions of the
Laplace-Beltrami operator. Accordingly, one may ask,
how many eigenfunctions are required in order to
embed a given manifold. In this talk, I will give
some background regarding the dimensionality
reduction problem, spectral geometry, and show
theoretical results for a generalized diffusion map.
Specifically, I will show a closed Riemannian
manifold can be embedded into a finite dimensional
Euclidean space by maps constructed based on the
connection Laplacian at a certain time. This time
and the embedding dimension can be bounded in terms
of the dimension and geometric bounds of the
manifold. Furthermore, the map based on heat kernels
can be made arbitrarily close to an isometry. In
addition, I will give a ‘’real world” example
pertaining to paleonthology, that demonstrates how
heat kernels and diffusion maps can be used to
quantify the similarity of shapes. The empirical
results suggest that this framework is better than
the metric commonly used in biological
morphometrics.

**February 12
**

**Christopher Henderson
**

**Title: Front slowdown due to nonlocal
interactions**

**Abstract:** Reaction-diffusion equations arise
as models of systems in which spreading and growing
forces interact in nontrivial ways, often creating a
front (i.e., a moving interface). In many
applications it is natural to consider nonlocal
interactions, but, mathematically, the resulting
equations have a number of new features and
technical difficulties; in particular, the
comparison principle, which implies that initially
ordered solutions remain ordered, no longer applies.
I will survey classical results, present several
examples of nonlocal reaction-diffusion equations,
and then focus on a particular one, the cane toads
equation, which is inspired by the invasive species
in Australia. In all cases, the emphasis will be on
the influence of long-range interactions due to
nonlocal terms on the behavior of fronts. In
particular, I will show that, surprisingly, the cane
toads front propagates slower than the standard
methods predict.

**February 14
**

**Maja Taskovic
**

**Title: Exponential tails for the Boltzmann
equation**

**Abstract:** In kinetic theory, a large system
of interacting particles is described by a particle
probability distribution function. One of the first
equations derived in such a way was the Boltzmann
equation (derived by Maxwell in 1866 and by
Boltzmann in 1872). The effect of collisions on the
density function is modeled by a bilinear integral
operator (collision operator) which in many cases
has a non-integrable angular kernel. For a long time
the equation was simplified by assuming that this
kernel is integrable with a belief that such an
assumption does not affect the equation
significantly. However, in last 20 years it has been
observed that a non-integrable singularity carries
regularizing properties, which motivates further
analysis of the equation in this setting.

We study the behavior in time of tails of solutions
to the Boltzmann equation in the non-cutoff regime
by examining the generation and propagation in time
of $L^1$ and $L^\infty$ exponentially weighted
estimates and the relation between them. For this
purpose we introduce Mittag-Leffler moments, which
can be understood as a generalization of exponential
moments. An interesting aspect of the result is that
the singularity rate of the angular kernel affects
the order of tails that can be propagated in time.
This is based on joint works with Alonso, Gamba, and
Pavlovic.

**February 18
**

**Florent Baudier
**

Texas A&M University

**Title**: Faithful embeddability
of graphs into Banach spaces and applications

**Abstract:**Faithful embeddability of graphs and metric spaces into Banach spaces is pivotal to research areas as diverse as:

-the design of approximation algorithms in theoretical computer science (sparsest cut problem, multi-commodity flows, approximate nearest neighbor search, sketching…),

-topology (Novikov conjecture),

-noncommutative geometry (coarse Baum-Connes conjecture),

-geometric group theory (Von Neumann’s amenability, Gromov’s program).

This non-exhaustive list can be stretched at will since metric spaces, with a wide variety of features, arise in nearly all areas of mathematics.

In this talk, I will focus on bi-Lipschitz and coarse embeddings of graphs (finite and infinite) into Banach spaces with some desirable geometric properties. I will discuss fundamental geometric problems of either local or asymptotic nature, in particular purely metric characterizations of “linear” properties of Banach spaces in the spirit of the Ribe program. One of the main goal of the talk is to present some fundamental ideas and techniques, as well as to convey the geometric intuition behind them.

**February 19
**

**Diane Holcomb
**

KTH Royal Institute of Technology

**Title**: On local point process
limits of random matrices

**Abstract:**The study of random matrices goes back to the works of Wishart (1920’s) and Wigner (1930’s). At the time they introduced a special class of models that had explicitly computable joint density functions for their eigenvalues. These joint densities turn out to be specific cases of a more general physical model called Coulomb gas models which describe particles interacting through some Hamiltonian. In this talk we will discuss a specific class of these Coulomb gas models called beta-ensembles which are a partial generalization of the random matrix eigenvalue process. I will begin by introducing a random matrix model and the generalization of its eigenvalue process to a beta-ensemble. We will then discuss how one can study the interactions of individual eigenvalues as the number of them grows to infinity. I will introduce the Sine-beta process, one of the limit processes that appears when the eigenvalues are rescaled to see these “local” interactions, and discuss several results and techniques that may be used for studying this process.

**February 25
**

**David Lipshutz
**

Technion - Israel Institute of Technology

**Title**: Sensitivity analysis of
reflected diffusions

**Abstract:**Reflected diffusions (RDs) constrained to remain in convex polyhedral domains arise in a variety of contexts, including as “heavy traffic” limits of queueing networks and in the study of rankbased interacting particle models. Sensitivity analysis of such an RD with respect to its defining parameters is of interest from both theoretical and applied perspectives. In this talk I will characterize pathwise derivatives of an RD in terms of solutions to a linear constrained stochastic differential equation whose coefficients, domain and directions of reflection depend on the state of the RD. I will demonstrate how pathwise derivatives are useful in Monte Carlo methods to estimate sensitivities of an RD, and also in characterizing sensitivities of the stationary distribution of an RD. The proofs of these results involve a careful analysis of sample path properties of RDs, as well as geometric properties of the convex polyhedral domain and the associated directions of reflection along its boundary.

**February 26
**

**Brooke Ullery
**

Harvard University

**Title**: **Cayley-Bacharach
theorems and polynomials vanishing on points
in projective space**

**Abstract:**If Z is a set of points in real n-space, we can ask which polynomials of degree d in n variables vanish at every point in Z. If P is one point of Z, the vanishing of a polynomial at P imposes one linear condition on the coefficients. Thus, the vanishing of a polynomial on all of Z imposes |Z| linear conditions on the coefficients. A classical question in algebraic geometry, dating back to at least the 4th century, is how many of those linear conditions are independent? For instance, if we look at the space of lines through three collinear points in the plane, the unique line through two of the points is exactly the one through all three; i.e. the conditions imposed by any two of the points imply those of the third. In this talk, I will survey several classical results including the original Cayley-Bacharach Theorem and Castelnuovo’s Lemma about points on rational curves. I’ll then describe some recent results and conjectures about points satisfying the so-called Cayley-Bacharach condition and show how they connect to several seemingly unrelated questions in modern algebraic geometry.

**February 28
**

**Achilles Beros
**

University of Hawaii

**Title**: Homogeneous
perfect binary trees

**Abstract:**Perfect binary trees typically have highly inhomogeneous branches; for example, if a perfect tree is computable, then for any set A there is a branch of higher degree than A. I will examine properties such that it is possible to construct perfect binary trees every branch of which have the property. Futhermore, I will show conditions on the frequency of branching that permit or preclude the construction of such a tree. I will also survey other recent research in computability theory and its applications.

**March 4
**

**Konstantin Slutsky
**

nuTonomy Asia

**Title: Orbit equivalences
of Borel flows**

**Abstract:**We provide an overview of the orbit equivalence theory of Borel flows. In general, an orbit equivalence of two group actions is a bijective map between phase spaces that maps orbits onto orbits. Such maps are often further required to posses regularity properties depending on the category of group actions that is being considered. For example, Borel dynamics deals with Borel orbit equivalences, ergodic theory considers measure-preserving maps, topological dynamics assumes continuity, etc.

Since its origin in 1959 in the work of H. Dye, the concept of orbit equivalence has been studied quite extensively. While traditionally larger emphasis is given to actions of discrete groups, in this talk we concentrate on free actions of Rn-flows while taking the viewpoint of Borel dynamics.

For a free Rn-action, an orbit can be identified with an “affine” copy of the Euclidean space, which allows us to transfer any translation invariant structure from Rn onto each orbit. The two structures of utmost importance will be that of Lebesgue measure and the standard Euclidean topology. One may than consider orbit equivalence maps that furthermore preserve these structures on orbits. Resulting orbit equivalences are called Lebesgue orbit equivalence (LOE) and time-change equivalence respectively.

Properties of LOE maps correspond most closely to those of orbit equivalence maps between their discrete counterparts — free Zn actions. We illustrate this by discussing the analog for Rn-flows of Dougherty-Jackson-Kechris classification of hyperfinite equivalence relations.

Orbit equivalences of flows often arise as extensions of maps between cross sections — Borel sets that intersect each orbit in a non-empty countable set. Furthermore, strong geometric restrictions on crosssections are often necessary. Following this path, we explain why one-dimensional R-flows posses cross sections with only two distinct distances between adjacent points, and show how this implies classification of R-flows up to LOE.

If time permits, we conclude the talk with an overview of time-change equivalence, emphasizing the difference between Borel dynamics and ergodic theory

**.**

**March 5
**

**Erik Slivken
**

University Paris VII

**Title: Large random
pattern-avoiding
permutations**

**Abstract:**

**A pattern in a permutation is a subsequence with a specific relative order. What can we say about a typical large random permutation that avoids a particular pattern? We use a variety of approaches. For certain classes we give a geometric description that relates these classes to other types of well-studied objects like random walks or random trees. Using the right geometric description we can find the the distribution of certain statistics like the number and location of fixed points. This is based on joint work with Christopher Hoffman and Douglas Rizzolo.**

**March 7
**

**Christopher
Porter
**

Ohio State University

**Title****:
Perspectives on the
transformation of random
sequences**** **

**Abstract:**

**The theory of algorithmic randomness provides a framework for studying the behavior of algorithmically random sequences under effective transformations, i.e. transformations that are Turing computable. In this talk, I will take as a starting point a theorem due to Demuth, which states that if we apply a total computable transformation to a sufficiently random sequence—here taken to be a Martin-Löf random sequence—then as long as the resulting sequence is not computable, one can recover an unbiased random sequence from the transformed sequence. I will then discuss the efficiency of the procedure that recovers unbiased randomness from this transformed sequence. More specifically, I will survey results on (1) the conditions for the existence of a computable bound on the number of input bits needed for this procedure to yield a given number of output bits (based on joint work with Laurent Bienvenu and subsequent work with Rupert Hölzl), and (2) finding the average-case extraction rate of this procedure (based on joint work with Douglas Cenzer).**

**March 12
**

**Dustin Mixon
**

Ohio State University

**Title****: Squeeze fit:
Label-aware dimensionality reduction by
semidefinite programming**** **

**Abstract:**Given labeled points in a high-dimensional vector space, we seek a low-dimensional subspace such that projecting onto this subspace maintains some prescribed distance between points of differing labels. Intended applications include compressive classification. This talk will introduce a semidefinite relaxation of this problem, along with various performance guarantees. (Joint work with Culver McWhirter (OSU) and Soledad Villar (NYU)).

**April 9
**

**Mark Lewis
**

Cornell University

**Title**: Constrained
Optimization for Scheduling in
Multi-Class Queueing

**Abstract:**
We consider the problem of scheduling
a single-server when there are
multiple parallel stations to serve. A
classic result in scheduling says to
create a station dependent index
consisting of the product of the
holding cost (per customer, per unit
time) times the rate at which the
service can be completed at that
station. The scheduler then
prioritizes work in the order of the
indices from highest to lowest.
Preferences are captured by the
various holding costs. A more natural
method for modeling preferences is to
assign constraints to the highest
priority customers (guaranteeing a
fixed quality of service level) and to
provide best effort service for the
other classes. We consider this
formulation, present conditions for
optimality and show how to construct
an optimal control. We initially focus
on the two station model, then explain
where the results can be extended.
Applications to patient flow in health
care are discussed.

**April 16
**

**Vladimir Chernyak
**

Wayne State University

**Title****: ****In****tegrability
in Time-Dependent Quantum Problems:
Factorization, Moduli Spaces, Spectral
Curves, and Representations of Quantum
Groups**

**Abstract:**

**Quantum evolution with time-dependent Hamiltonians, $i\hbar \dot{\Psi}(t) = {H}(t) \Psi(t)$, as of today draws considerable attention, both in experimental and theoretical research. The simplest model with H(t) = A + Bt, A and B being N x N real hermitian matrices, has a known exact solution in special functions only for N=2, and is known as the multi-level Landau Zener problem (MLZ). However, for a certain class of N-dimensional problems that satisfy phenomenologically determined ``integrability'' conditions, the scattering matrix can be represented in a factorized form, with the elementary scattering events being represented in terms of known 2 x 2 scattering matrices. We reveal the reason that stands behind the aforementioned factorization: Each integrable MLZ problem can be embedded into a system of M linear first-order differential equations with respect to M-dimensional time, that satisfy consistency constraints. Classification of exactly solvable MLZ problems leads to moduli spaces that can be mapped into moduli spaces of smooth complex curves. We will speculate on possible relevance to number theory, and in particular modular forms. Finally we consider a more general, still integrable, so-called BCS model that describes N_ s identical quantum spins, with H(t) = H_C t^{-1} + H_G. It can be efficiently treated by making use of symmetry with respect to Artin's Braid Group B_{N_s}, due to effects of monodromy in the extended space. The compact Quantum Group SU_{q} (2) appears naturally as an auxiliary tool for describing relevant representation of the Braid group.**

**May 3 (New Room and Time: Carver 268
at 3:10 p.m.)
**

**Adriana Salerno
**

Bates College

**Title**: Hypergeometric decomposition of symmetric K3 quartic pencils

**Abstract**

**:**In this talk, we will show the hypergeometric functions associated to five one-parameter deformations of Delsarte K3 quartic hypersurfaces in projective space. We compute all of their Picard–Fuchs differential equations; we count points using Gauss sums and rewrite this in terms of finite field hypergeometric sums; then we match up each differential equation to a factor of the zeta function, and we write this in terms of global L-functions. This computation gives a complete, explicit description of the motives for these pencils in terms of hypergeometric motives. This is joint work with Charles F Doran, Tyler L Kelly, Steven Sperber, John Voight, and Ursula Whitcher.

**Fall 2018**

**Tuesdays 4:10 p.m. in**

__Hoo____ver 1213__- Tea and cookies starting at 3:45 p.m. in Carver 404

**September 7 (Friday at 3:10pm - Carver 305 -
Colloquium / Discrete Math seminar)
**

**Shira Zerbib
**

University of Michigan

**Title**: Colorful coverings of
polytopes -- the hidden topological truth behind
different colorful phenomena

**Abstract: **The
topological KKMS theorem, a powerful extension of
Brouwer's Fixed-Point theorem, was proved by Shapely
in 1973 in the context of game theory. We prove a
colorful and polytopal generalization of the KKMS
Theorem, and show that our theorem implies some
seemingly unrelated results in discrete geometry and
combinatorics involving colorful settings. For
example, we apply our theorem to provide a new proof
of the Colorful Caratheodory Theorem due to Barany.
We further apply our theorem to obtain a new upper
bound on the piercing numbers in colorful
$d$-interval families, extending results of Tardos,
Kaiser and Alon for the non-colored case. Finally,
we apply our theorem to questions regarding fair
division.

Joint with Florian Frick.

**September 11
**

**Maja Taskovic
**

University of Pennsylvania

**Title**: On the relativistic
Landau equation

**Abstract:**In 1936, Landau derived a model for a dilute hot plasma where fast moving particles interact via Coulomb interactions. Instead of tracking every particle separately, which would lead to a large number of equations, the dynamics is being described by the particle density function. This model, also known as the Landau equation, does not include the effects of Einstein's theory of special relativity. However, when particle velocities are close to the speed of light, which happens frequently in a hot plasma, then relativistic effects become important. A model that captures these effects, the relativistic Landau equation, was derived by Budker and Beliaev in 1956.

We study the Cauchy problem for the spatially homogeneous relativistic Landau equation with Coulomb interactions. The difficulty of the problem lies in the extreme complexity of the kernel of the collision operator. We present a new decomposition of such kernel. This is then used to prove the global Entropy dissipation estimate, the propagation of any polynomial moment for a weak solution, and the existence of a true weak solution for a large class of initial data.

This is joint work with Robert M. Strain.

**September 18
**

**Krishna B. Athreya
**

Iowa State University

**Title**: David Harold Blackwell

**Abstract:**David Harold Blackwell was a great mathematician, statistician that belonged to the African American Community from Central Illinois. He was born in 1919 and died in 2010. He got his PhD in mathematics from University of Illinois in 1941 under the great American mathematician J. L. Doob. David Blackwell worked in many areas in mathematics. These include Probability Theory, Game Theory, Information Theory and Bayesian Analysis. He was the first African American to be elected to full professorship at UC Berkeley. He was a member of the US Academy of Sciences, Fellow of the UK Royal Statistical Society, and received many other honors. In this talk we shall outline some of his major research contributions.

**September 25
**

**Caroline Terry
**

University of Chicago

**Title**: A stable arithmetic
regularity lemma in finite-dimensional vector spaces
over fields of primer order

**Abstract:**Arithmetic combinatorics studies the additive and multiplicative structure of subsets of groups, especially finite abelian groups such as cyclic groups of prime order, or finite dimensional vector spaces over finite fields. Insipired by Szemerèdi's regularity lemma, arithmetic regularity lemmas are tools used in arithmecit combinatorics to produce group theoretic analogues of results from graph theory. The arithmetic regularity lemma for $F_p^n$ (first proved by Green in 2005) states that given $A\subseteq F_p^n$, there exists $H\leq F_p^n$ of bounded index such that $A$ is Fourier-uniform with respect to almost all cosets of $H$. In general, the growth of the codimension of H is required to be of tower type depending on the degree of uniformity, and must also allow for a small number of non-uniform elements. The main result of this talk is that, under a natural model theoretic assumption called stability, the bad bounds and non-uniform elements are not necessary. Specifically, we present an arithmetic regularity lemma for $k$-stable sets $A\subseteq F_p^n$, where the bound on the codimension of the subspace is only polynomial in the degree of uniformity, and where there are no non-uniform elements. This result is a natural extension to the arithmetic setting of the work on stable graph regularity lemmas initiated by Malliaris and Shelah.

This is joint work with Julia Wolf.

** October 2
**

**Tuncay Aktosun
**

The University of Texas at Arlington

**Title**: Determining the shape of
a human vocal tract from speech sounds

**Abstract:**The elementary units for human speech are called phonemes, and the utterance of each phoneme by a person is governed by a particular shape of that person's vocal tract. A mathematical description is presented for the shape of the vocal tract during the creation of each phoneme, which corresponds to the direct problem. Then, a corresponding inverse problem is analyzed; namely, the determination of the shape of the human vocal tract from the sound pressure measurements at the mouth associated with an uttered phoneme.

The talk is based on joint work with P. Sacks of Iowa State University.

**October 9
**

**Naiomi Cameron
**

Clark College

**Title**: Inversion generating
functions for signed pattern avoiding permutations

**Abstract:**We consider the classical Mahonian statistics on the set $B_n(\Sigma)$ of signed permutations in the hyperoctahedral group $B_n$ which avoid all patterns in $\Sigma$, where $\Sigma$ is a set of patterns of length two. In 2000, Simion gave the cardinality of $B_n(\Sigma)$ in the cases where $\Sigma$ contains either one or two patterns of length two and showed that $\left|B_n(\Sigma)\right|$ is constant whenever $\left|\Sigma\right|=1$, whereas in most but not all instances where $\left|\Sigma\right|=2$, $\left|B_n(\Sigma)\right|=(n+1)!$. We answer an open question of Simion by providing bijections from $B_n(\Sigma)$ to $S_{n+1}$ in these cases where $\left|B_n(\Sigma)\right|=(n+1)!$. In addition, we extend Simion's work by providing a combinatorial proof in the language of signed permutations for the major index on $B_n(21, \bar{2}\bar{1})$ and by giving the major index on $D_n(\Sigma)$ for $\Sigma =\{21, \bar{2}\bar{1}\}$ and $\Sigma=\{12,21\}$. The main result of this paper is to give the inversion generating functions for $B_n(\Sigma)$ for almost all sets $\Sigma$ with $\left|\Sigma\right|\leq2.$

**October 16
**

**Vlad Vicol
**

Courant Institute

**Title**: Nonuniqueness of weak
solutions to the Navier-Stokes equation

**Abstract:**We prove that distributional solutions of the 3D Navier-Stokes equations are not unique in the class of weak solutions with finite kinetic energy. Moreover, we prove that Hölder continuous weak solutions of the 3D Euler equations may be obtained as a strong vanishing viscosity limit of a sequence of finite energy weak solutions of the 3D Navier-Stokes equations.

This is a joint work with Tristan Buckmaster.

**October 23
**

**Eric Weber
**

Iowa State University

**Title**: The Kaczmarz algorithm:
theory and applications

**Abstract:**The Kaczmarz algorithm is an iterative method for solving systems of linear equations that was introduced by Stefan Kaczmarz in 1937. The algorithm is now enjoying a resurgence in interest, as it has been found useful in data science applications. It also has remarkably deep connections to complex and harmonic analysis. We shall introduce the algorithm, demonstrate some of its features, and present some of its applications. Towards the end of the talk, we shall outline these deep connections to complex and harmonic analysis.

**October 30
**

**Brooke Ullery
**

Harvard University

**Title**: Measures of
irrationality for algebraic varieties

**Abstract:**In algebraic geometry, a smooth curve is said to be rational if it is isomorphic to $P^1$, the projective line. More generally, the gonality of a smooth projective curve is the smallest degree of a map from the curve to the projective line. There are a few different definitions that attempt to generalize the notion of gonality even further to higher dimensional varieties. The intuition is that the higher these numbers, the further the variety is from being rational. We will discuss these measures of irrationality and various methods of calculating and bounding them. We will mainly focus on the examples of hypersurfaces and, more generally, complete intersections in projective space. All of these terms will be defined and there will be lots of accessible examples!

**November 2 (Friday at 3:10pm - Carver 305 -
Colloquium / Discrete Mathematics seminar
- Tea and cookies will be
afterwards, at 4:00pm in Carver 404)
**

**Emily Sergel
**

University of Pennsylvania

**Title**: Parking functions,
schedules, and the Delta Conjecture

**Abstract:**A parking function is a kind of labeled lattice path. The Shuffle Conjecture states that a certain enumeration of parking functions is closely related to an important symmetric function basis called the Macdonald polynomials. The Delta Conjecture is a generalization with similar ties to Macdonald polynomials. We discuss the combinatorics of parking functions and the notion of schedules as they pertain to these two settings.

Joint work with Jim Haglund.

**November 6
**

**Marta D'Elia
**

Sandia National Laboratories

**Title: **Nonlocal models in
computational science and engineering: challenges
and applications

**Abstract: **Nonlocal
continuum theories such as peridynamics and nonlocal
elasticity can capture strong nonlocal effects due
to long-range forces at the mesoscale or microscale.
For problems where these effects cannot be
neglected, nonlocal models are more accurate than
classical Partial Differential Equations (PDEs) that
only consider interactions due to contact. However,
the improved accuracy of nonlocal models comes at
the price of a computations cost that is
significantly higher than that of PDEs.

In this talk I will present nonlocal models and
the Nonlocal Vector Calculus, a theory that allows
one to treat nonlocal diffusion problems in almost
the same way as PDEs. Furthermore, I will present
current open challenges related to the numerical
solution of nonlocal problems and show how we are
currently addressing them. Specifically, I will
describe an optimization-based local-nonlocal
coupling strategy and briefly introduce a technique
to improve the performance of Finite Element (FE)
approximations.

The goal of local-nonlocal coupling methods is
to combine the computational efficiency of PDEs with
the accuracy of nonlocal models. These couplings are
imperative when the size of the computational domain
or the extent of the nonlocal interactions are such
that the nonlocal solution becomes prohibitively
expensive to compute, yet the nonlocal model is
required to accurately resolve small scale features.
Our approach formulates the coupling as a control
problem where the states are the solutions of the
nonlocal and local equations, the objective is to
minimize their mismatch on the overlap of the
nonlocal and local domains, and the controls are
virtual volume constraints and boundary conditions.
I will present consistency and convergence studies
and, using three-dimensional geometries, I will also
show that our approach can be successfully applied
to challenging, realistic problems.

**November 13
**

**Catherine Searle
**

Wichita State University

**Title**: Symmetries of spaces
with lower curvature bounds

**Abstract:**The classification of manifolds of positive and non-negative sectional curvature is a long standing problem in Riemannian geometry. In particular, restricting our attention to closed, simply-connected manifolds, there are no topological obstructions that allow us to distinguish between positive and non-negative curvature, that is, we have no examples of manifolds that admit a metric of non-negative curvature that do no admit a metric of positive curvature. However, with the introduction of symmetries, we are able to distinguish between these two classes. In this context, I will discuss recent joint work with Christine Escher and work with Zheting Dong and Christine Escher on non-negatively curved manifolds with abelian symmetries.

**December 12
**

**Shira Zerbib
**

**December 13
**

**Shlomo Gelaki
**