# Events This Week

Week of April 23 - April 19

### Wednesday, April 21

Dissertation Defense

Topic: Qualitative analysis of a PDE model for chemotaxis with logarithmic sensitivity and logistic growth

Padi Fuster Aguilera | Tulane University

Abstract: This thesis examines the qualitative behaviour of solutions to a PDE model for chemotaxis; that is the existence, uniqueness and asymptotic behavior of solu- tions. We study the initial-boundary value problem for a repulsive chemotaxis model with logarithmic sensitivity and logistic growth for the cell population density, and nonlinear growth of the chemical concentration. Extensive work has been done for this particular model without logistic growth on both bounded and unbounded domains. However, the model with logistic growth on a bounded domain has not been studied before. This case is of particular interest given its relevance for modeling tumor angiogenesis. We first establish global well-posedness of strong solutions for large initial data with no-flux boundary conditions and, moreover, establish the qualitative results that both the population density and chemical concentration asymptotically converge to constant states. The population density in particular con- verges to its carrying capacity. We additionally prove that the vanishing chem- ical diffusivity limit holds in this regime. Finally, we provide numerical confirmation of the rigorous qualitative results, as well as numerical simulations that demonstrate a separation of scales phenomenon. We then establish global well-posedness of strong solutions for large initial data with dynamic boundary conditions. Moreover, the solutions will asymptotically approach the boundary data under mild and natural assumptions on the boundary functions. We additionally show the formation of a boundary pro- file in the singular chemical zero diffusive limit. Lastly, we provide numerical simulations that confirm the boundary layer formation, as well as convergence towards certain steady states of the solution when relaxing the assumptions on the boundary data. The main tool developed in these results is a particular Lyapunov functional that helps overcome the mathematical challenges of the non-conservation of the mass due to the logistic growth. These results give a complete study of this particular system on bounded domains with both zero-flux and dynamic moving boundary conditions.

Zoom access
Time: 1:00

Week of April 16 - April 12

### Friday, April 16

Applied and Computational Mathematics

Topic: TBA

Chia Ying Lee | Motorola Solutions

Abstract: TBA

Zoom access
Time: 3:30

### Thursday, April 15

Colloquium

Topic: Analysis of free boundary problems in fluid mechanics

Ian Tice - Carnegie Mellon University (Host: Glatt-Holtz)

Abstract: A free boundary problem in fluid mechanics is one in which the fluid domain is not specified a priori and evolves in time with the fluid.  Such problems are ubiquitous in nature and occur at a huge range of scales, from dew drops, to waves on the ocean, to the surface of a star.  In this talk I will review the basic features of free boundary problems and how we incorporate interesting interfacial physics effects.  I will also survey recent work on various viscous free boundary problems.

Zoom accessContact mbrown2@math.tulane.edu
Time: 3:30

### Wednesday, April 14

Algebra and Combinatorics Seminar

Topic: Nodes on Quintic Spectrahedra

Taylor Brysiewicz | MPI Leipzig

Abstract: A spectrahedron in R3 is the intersection of a 3-dimensional affine linear subspace of dxd real matrices with the cone of positive-semidefinite matrices. Its algebraic boundary is a surface of degree d in C3 called a symmetroid. Generically, symmetroids have (d^3-d)/6 nodes over C and the real singularities are partitioned into those which lie on the spectrahedron and those which do not. This data serves as a coarse combinatorial description of the spectrahedron. For d=3 and 4, the possible partitions are known. In this talk, I will explain how we determined which partitions are possible for d=5. In particular, I will explain how we used numerical algebraic geometry and an enriched hill-climbing algorithm to find explicit examples of spectrahedra witnessing each partition.

Zoom access

Meeting ID:
Zoom meeting starts at 3:00pm

### Tuesday, April 13

Geometry and Topology

Topic: AATRN: High-dimensional data, level-set  geometry, and Voronoi analysis of spatial point sets

Menachem Lazar (Bar-Ilan University)

Abstract: Physical systems are regularly  studied as spatial point sets, and so understanding the structure of such sets is a very natural problem.  However, aside from special cases, describing the manner in which a set of points is arranged in space can be quite challenging.  In the first part of  this talk, I will show how consideration of the configuration space of local arrangements of neighbors can shed light on essential challenges of this problem, and in the classification of high-dimensional data more generally.  In the second part of the talk  I will introduce some ideas from Voronoi cell topology and show how they can be used to define crystals, defects, and order more generally in a somewhat precise manner.

Recordings of most of the talks will be posted to the AATRN YouTube Channel.

Zoom access:
Meeting ID: 924  8568 4682
Zoom meeting starts at 10:00 CT

Week of April 9 - April 5

### Friday, April 9

Applied and Computational Mathematics

Topic: Transition from academia to industry

Camelia Pop | TBA

Abstract: I plan to speak about my transition from an academic career to one in the financial industry. I will talk about my background, how I prepared for this change, and the similarities and differences between the two career paths, from my experience.

Zoom access
Time: 3:30

### Thursday, April 8

Colloquium

Topic: Mixing, transport, and enhanced dissipation

Anna Mazzucato | Penn State (Host: Glatt-Holtz)

Abstract: I will discuss transport of passive scalars by incompressible flows and measures of optimal mixing and stirring.  I will present two examples of opposite effects of mixing: one leads to irregular transport and a dramatic, instantaneous loss of regularity for transport equation, the other is enhanced dissipation, which can lead to global existence in non-linear, dissipative systems. In particular, I will  show how mixing leads to global existence for the 2D Kuramoto-Sivashisky equation, a model for flame propagation.

Zoom access: Contact mbrown2@math.tulane.edu
Time: 3:30pm

### Wednesday, April 7

Geometry and Topology

Topic: AATRN: How optimal transport can help us to determine the curvature of complex networks?

Marzieh Eidi | Max Planck

Abstract: Ollivier Ricci curvature  is a notion that originated from Riemannian Geometry and is suitable for applying on different settings from smooth manifolds to discrete structures such as (directed) hypergraphs. In the past few years, alongside Forman Ricci curvature, this curvature as  an edge-based measure has become a popular and powerful tool for network analysis. This notion is defined based on optimal transport problem (Wasserstein distance) between sets of probability measures supported on data points and can nicely detect some important  features such as clustering and sparsity in their structures. After introducing this notion for (directed) hypergraphs and mentioning some of its properties, as one of the main recent applications, I will present the result of the implementation of this tool  for the analysis of chemical reaction networks.

Recordings of most of the talks will be posted to the AATRN YouTube Channel.

Zoom access:
Meeting ID: 924  8568 4682
Zoom meeting starts at 10:00 CT

Week of April 2 - March 29

### Friday, April 2

Applied and Computational Mathematics

Topic: Learning Temporal Evolution of Spatial Dependence

Shiwei Lan | Arizona State University School of Mathematical and Statistical Sciences

Abstract: We are living in an era of data explosion usually featured with big data' or big dimension'. However, there is another big challenge in data science that we cannot ignore - complex relationship. Spatiotemporal data are ubiquitous in our life and have been a trending topic in the scientific community, e.g. the dynamic brain connectivity study in neuroscience. There is usually complicated dependence among spatial locations and such relationship does not necessarily stay static over time. The temporal evolution of spatial dependence (TESD) is often of scientific interest in understanding the underlying mechanism behind natural phenomena such as cognition and disease progression.

In this talk, I will introduce two novel statistical methods to learn TESD in various applications. The first is a semi-parametric method modeling TESD as dynamic covariance matrices [1]. A spherical product representation of covariance matrix is introduced to ensure its positive-definiteness along the process. An efficient MCMC algorithm based on the representation is implemented for Bayesian inference. The second is a fully nonparametric generalization of the first model based on spatiotemporal Gaussian process (STGP) [2]. It further enables scientists to extend the learned TESD to new territory where there are no data. While classic STGP with a covariance kernel separated in space and time fails in this task, I propose a novel generalization to introduce the time-dependence to the spatial kernel that can effectively and efficiently characterize TESD. The utility and advantage of the proposed methods will be demonstrated by a number of simulations, a study of dynamic brain connectivity and a longitudinal neuroimaging analysis of Alzheimer's patients.

Zoom access
Time: 3:30

### Wednesday, March 31

Algebra and Combinatorics Seminar

Topic: Uniform Asymptotic Growth of Symbolic Powers of Ideals

Robert Walker | University of Wisconsin-Madison

Abstract: Algebraic geometry (AG) is a major generalization of linear algebra which is fairly influential in mathematics. Since the 1980's with the development of computer algebra systems like Mathematica, AG has been leveraged in areas of STEM as diverse as statistics, robotic kinematics, computer science/geometric modeling, and mirror symmetry. Part one of my talk will be a brief introduction to AG, to two notions of taking powers of ideals (regular vs symbolic) in Noetherian commutative rings, and to the ideal containment problem that I study in my thesis. Part two of my talk will focus on stating the main results of my thesis in a user-ready form, giving a "comical" example or two of how to use them. At the risk of sounding like Paul Rudd in Ant-Man, I hope this talk will be awesome.

Zoom access

Meeting ID:
Zoom meeting starts at 3:00pm

### Wednesday, March 31

Geometry and Topology

Topic: AATRN: Graph representation learning and its applications to biomedicine

Marinka Zitnik | Harvard University

Abstract: he success of machine learning  depends heavily on the choice of representations used for prediction tasks. Graph representation learning has emerged as a predominant choice for learning representations of networks. In this talk, I describe our efforts to expand the scope and ease the applicability  of graph representation learning. First, I outline SubGNN, a subgraph neural network for learning disentangled subgraph embeddings. SubGNN generates embeddings that capture complex subgraph topology, including structure, neighborhood, and position of subgraphs  in a graph. Second, I will discuss applications in biology and medicine. The new methods predicted disease treatments that were experimentally verified in the wet laboratory. Further, the methods helped to discover dozens of combinations of drugs safe for  patients with considerably fewer unwanted side effects than today's treatments. Lastly, I describe our efforts in learning actionable representations that allow users of our models to receive predictions that can be interpreted meaningfully.

Recordings of most of the talks will be posted to the AATRN YouTube Channel.

Zoom access:
Meeting ID: 924  8568 4682
Zoom meeting starts at 10:00 CT

### Tuesday, March 30

Topic: The Representation Theory of Angular Momentum in Quantum Mechanics

Abstract: Often, the theory of Lie groups, Lie algebras, and their representations make an appearance in physics. The underlying symmetries responsible for this appearance are exploited by physicists to reduce the complexity of a given problem. For example, the rotational symmetry in the Hydrogen atom is used to split the 3-dimensional problem of finding the eigenvalues of the associated Hamiltonian into two parts: finding a solution for the angular part (a 2-dimensional problem) and finding a solution for the radial part (1-dimensional). These solutions are then pieced back together. In this talk, I will discuss the theory of angular momentum in quantum mechanics from the perspective of the representation theory of Lie groups and Lie algebras.

Zoom access: 954 0971 2190
Time: 5:00pm

Week of March 26 -  March 22

### Friday, March 26

Applied and Computational Mathematics

Topic: Diffuse Interface modeling for two-phase flows: the journey from the model H to the AGG model

Andrea Giorgini | Indiana University

Abstract: In the last decades, the Diffuse Interface theory (also known as Phase Field theory) has made significant progresses in the description of multi-phase flows from modeling to numerical simulations. A particularly active research topic has been the development of thermodynamically consistent extensions of the well-known Model H in the case of unmatched fluid densities. In this talk, I will focus on the AGG model proposed by H. Abels, H. Garcke and G. Grün in 2012. The model consists of a Navier-Stokes-Cahn-Hilliard system characterized by a concentration-dependent density and an additional flux term due to interface diffusion. Using the method of matched asymptotic expansions, it was shown that the sharp interface limit of the AGG model corresponds to the two-phase Navier-Stokes equations. In the literature, the analysis of the AGG system has only been focused on the existence of weak solutions. During the seminar, I will present the first results concerning the existence, uniqueness and stability of strong solutions for the AGG model in two dimensions.

Zoom access

Meeting ID:
Zoom meeting starts at 3:30pm

### Friday, March 26

Geometry and Topology

Topic: AATRN: Vietoris-Rips Persistent Homology, Injective Metric Spaces, and The Filling Radius

Facundo Mémoli | Ohio State

Abstract: The persistent homology induced  by the Vietoris-Rips simplicial filtration is a standard method for capturing topological information from metric spaces. We consider a different, more geometric way of generating persistent homology of metric spaces which arises by first embedding a given  metric space X into a larger ambient metric space E and then considering neighborhoods of the original space X inside E.

We then prove that the persistent homology  of the Vietoris-Rips filtration is isomorphic to our geometric persistent homology provided that the ambient metric space E satisfies a property called injectivity.

As an application of this isomorphism  result we are able to precisely characterize the type of intervals that appear in the persistence barcodes of the Vietoris-Rips filtration of any compact metric space and also to give succinct proofs of the characterization of the persistent homology of products  and metric gluings of metric spaces.

Our results also permit proving several  bounds on the length of intervals in the Vietoris-Rips barcode by other metric invariants.

As another application, we connect  this geometric persistent homology to the notion of filling radius of manifolds introduced by Gromov and show some consequences related to (1) the homotopy type of the Vietoris-Rips complexes of spheres which follow from work of M. Katz and (2) characterization  (rigidity) results for spheres in terms of their Vietoris-Rips persistence barcodes which follow from work of F. Wilhelm.

Zoom access

Meeting ID:
Zoom meeting starts at 10:00 CT

### Wednesday, March 24

Algebra and Combinatorics Seminar

Topic: Quantum Groups, R-Matrices, Quantum Yang-Baxter Equation and Solvable Lattice Models

Mike Joyce | Tulane University

Abstract: In this talk, we will briefly define quantum groups and focus on one specific example, quantum sl_2. Quantum group modules yield interesting R-matrices that arise from the almost cocommutativity of the quantum group. These R-matrices satisfy the Quantum Yang-Baxter Equation (QYBE). We'll connect this to an area of intense current research, solvable lattice models, which have applications in algebraic combinatorics, number theory, and probability. We'll see how quantum sl_2 explains properties of the six vertex model, the simplest and most well-understood solvable lattice model.

Zoom access
Time: 3:00

### Tuesday, March 23

Topic: Convex sets associated to algebraic objects

Thai Nguyen | Tulane University

Abstract: I will talk about a way to associate convex sets to polynomials and ideals in polynomial rings. This idea can be traced back to an idea of Issac Newton (~1676) that was used to prove Newton-Puiseux theorem. In 1996, Okounkov introduced a way to associate convex sets to algebraic varieties in order to show the log-concavity of the degrees of those algebraic varieties. Later on, this construction was studied systematically by the works of Kaveh-Khovanskii and Lazarsfeld-Mustata (~2009). It has become a very active research area recently and is now known as the theory of Newton-Okounkov bodies. Among numerous applications of this theory, I will present a beautiful one, which is to count the number of solutions of a system of (general) polynomials (so-called Kouchnirenko-Bernstein theorem that was proved many years ago). I will also discuss how this relates to my current project, involving Newton polyhedra and symbolic polyhedra.

Zoom access: 954 0971 2190
Time: 5:00pm

Week of March  19 -  March 15

### Friday, March 19

Applied and Computational Mathematics

Topic: Monte Carlo methods for the Hermitian eigenvalue problem

Robert Webber | NYU-Courant

Abstract: In quantum mechanics and the analysis of Markov processes, Monte Carlo methods are needed to identify low-lying eigenfunctions of dynamical generators. The standard Monte Carlo approaches for identifying eigenfunctions, however, can be inaccurate or slow to converge. What limits the efficiency of the currently available spectral estimation methods and what is needed to build more efficient methods for the future? Through numerical analysis and computational examples, we begin to answer these questions. We present the first-ever convergence proof and error bounds for the variational approach to conformational dynamics (VAC), the dominant method for estimation eigenfunctions used in biochemistry. Additionally, we analyze and optimize variational Monte Carlo, which combines Monte Carlo with neural networks to accurately identify low-lying eigenstates of quantum systems.

Zoom access

Meeting ID:
Zoom meeting starts at 3:30pm

### Wednesday, March 17

Algebra and Combinatorics Seminar

Topic: On combinatorics of Arthur's trace formula, convex polytopes, and toric varieties

Kiumars Kaveh | University of Pittsburgh

Abstract: I start by discussing two beautiful well-known theorems about decomposing a convex polytope into an signed sum of cones, namely the classical Brianchon-Gram theorem and Lawrence-Varchenko theorem. I will then explain a generalization of the Brianchon-Gram which can be summerized as "truncating a function on the Euclidean space with respect to a polytope". This is an extraction of the combinatorial ingredients of Arthur's ''convergence'' and ''polynomiality'' results in his famous trace formula. Arthur's trace formula concerns the trace of left action of a reductive group G on the space L^2(G/Γ) where Γ is a discrete (arithmetic) subgroup. The combinatorics involved is closely related to compactifications of ''locally summetric spaces'' (which btw are hyperbolic manifolds). Our ''combinatorial truncation'' can be thought of as an analogue of Arthur's truncation over a toric variety (in place of a compactification of a locally symmetric space). This is joint work in progress with Mahdi Asgari (Oklahoma State).

Zoom access
Time: 3:30

### Tuesday, March 16

Topic: Ulam´s problem

John Lopez | Tulane University

Abstract: In this talk we will take a look to Ulam´s problem, which is related to the length (l_n) of the longest increasing subsequence in a random permutation of {1, 2, 3,...,n}. This question has been studied by a variety of increasingly technically sophisticated methods over the last 30 years. We will study the asymptotic behavior of the expected value of l_n, (E(l_n)), by analyzing the limit of E(l_n)/sqrt(N). Specifically, we will derive a lower bound for this limit using some combinatorial arguments and will also introduce some probabilistic tools which are used to determine its existence.

Zoom access: 954 0971 2190
Time: 5:00pm

Week of March  12 -  March 8

### Friday, March 12

Applied and Computational Mathematics

Topic: Is MCMC Really Slower Than Variational Inference?

Abstract: Variational inference (VI) and Markov chain Monte Carlo (MCMC) are approximate posterior inference algorithms that are often said to have complementary strengths, with VI being fast but biased and MCMC being slower but asymptotically unbiased. We analyze gradient-based MCMC and VI procedures and find theoretical and empirical evidence that these procedures are not as different as one might think. In particular, a close examination of the Fokker- Planck equation that governs the Langevin dynamics (LD) MCMC procedure reveals that LD implicitly follows a gradient flow that corresponds to a VI procedure based on optimizing a nonparametric normalizing flow. This result suggests that the transient bias of LD (due to the Markov chain not having burned in) may track that of VI (due to the optimizer not having converged), up to differences due to VI’s asymptotic bias and parameterization. Empirically, we find that the transient biases of these algorithms (and their momentum-accelerated counterparts) do evolve similarly. This suggests that practitioners with a limited time budget may get more accurate results by running an MCMC procedure (even if it doesn't quite converge) than a VI procedure, as long as the variance of the MCMC estimator can be dealt with (e.g., by running many parallel chains on a GPU). I will also briefly discuss ChEES-HMC, an adaptive Hamiltonian Monte Carlo method that is better suited to GPU parallelization than the widely used NUTS algorithm

Zoom access

Meeting ID:
Zoom meeting starts at 3:30pm

### Friday, March 12

Geometry and Topology

Topic: Vietoris-Rips thickenings: Problems for birds and frogs

Abstract: An artificial distinction is to describe some mathematicians as birds, who from their high vantage point connect disparate areas of mathematics through broad theories, and other mathematicians as frogs, who dig deep into particular problems to solve them one at a time. Neither type of mathematician thrives without the help of the other! In this talk, I will survey open problems related to Vietoris-Rips complexes that are attractive to both birds and frogs. Though Vietoris-Rips complexes are frequently used to approximate the shape of a dataset, many questions remain about their mathematical properties. Frogs may delight in open problems such as the homotopy types of Vietoris-Rips complexes of spheres, ellipsoids, tori, graphs, Cayley graphs of groups, geodesic spaces, subsets of the plane, and even the integer lattice Z^n with the taxicab metric for n >= 4. Birds may enjoy emerging connections between Vietoris-Rips complexes and a variety of areas in pure mathematics, including metric geometry (Gromov-Hausdorff distances), quantitative topology (Gromov's filling radius), measure theory (optimal transport), topological combinatorics (Borsuk-Ulam theorems), geometric group theory (finiteness properties of groups), and geometric topology (thick-thin decompositions).

Zoom access

Meeting ID:
Zoom meeting starts at 10:00 CT

### Wednesday, March 10

Algebra and Combinatorics Seminar

Topic: When is a (projectivized) toric vector bundle a Mori dream space?

Christopher Manon | University of Kentucky

Abstract: Like toric varieties, toric vector bundles are a rich class of varieties which admit a combinatorial description. Following the classification due to Klyachko, a toric vector bundle is captured by a subspace arrangement decorated by toric data. This makes toric vector bundles an accessible test-bed for concepts from algebraic geometry. Along these lines, Hering, Payne, and Mustata asked if the projectivization of a toric vector bundle is always a Mori dream space. Su\ss and Hausen, and Gonzales showed that the answer is "yes" for tangent bundles of smooth, projective toric varieties, and rank 2 vector bundles, respectively. Then Hering, Payne, Gonzales, and Su\ss showed the answer in general must be "no" by constructing an elegant relationship between toric vector bundles and various blow-ups of projective space, in particular the blow-ups of general arrangements of points studied by Castravet, Tevelev and Mukai. In this talk I'll review some of these results, and then show a new description of toric vector bundles by tropical information. This description allows us to characterize the Mori dream space property in terms of tropical and algebraic data. I'll describe new examples and non-examples, and pose some questions. This is joint work with Kiumars Kaveh.

Zoom access: Contact mbrown2@math.tulane.edu
Time: 3:30pm

Week of March  5 -  March 1

### Friday, March 5

Applied and Computational Mathematics

Topic: TBA

Nitsan Ben-Gal | 3M

Abstract: TBA

Zoom access

Meeting ID:
Zoom meeting starts at 3:30pm

### Thursday, March 4

Colloquium

Topic: TBA

Mark Girolami | Cambridge (Host: Glatt-Holtz)

Abstract: TBA

Zoom access: Contact mbrown2@math.tulane.edu
Time: 3:30pm

### Wednesday, March 3

Algebra and Combinatorics Seminar

Topic: A Survey of Hopf Algebras and Quantum Groups

Mike Joyce | Tulane University

Abstract: We will introduce Hopf algebras and quantum groups through some of their key properties and some of the simplest examples. We will discuss R-matrices and the Yang-Baxter equation and then survey some of their manifestations in other areas of mathematics.

Zoom access:
Time: 3:30

### Tuesday, March 2

Topic: An Intro to Knot Invariants

Will Tran | Tulane University

Abstract: The word "knot" and phrase "knot theory" have been intimidating students since the early 1900s. We may see the word "knot" and immediately shut down, thinking "I don't know enough topology to study knots." It turns out that understanding knot invariants -- a primary focus in knot theory -- requires little to no topology at all! In fact, with some undergraduate linear algebra and an open mind, you too can get started on understanding knot theory. In this talk, we'll compute elementary knot invariants such as p-colorability, the knot determinant, and the Alexander Polynomial. If time permits, we'll see how other knot invariants like the Gauss Linking Integral, Linking Number, and Moebius Energy can connect to number theory, algebra, analysis, biology, and physics.

Meeting ID: 954 0971 2190
Time: 5:00pm

Week of February  26 -  February 22

### Friday, February 26

Applied and Computational Mathematics

Topic: An examination of school reopening strategies during the SARS-CoV-2 pandemic

Alfonso Landeros | UCLA

Abstract: The SARS-CoV-2 pandemic led to closure of nearly all K-12 schools in the United States of America in March 2020. Although reopening K-12 schools for in-person schooling is desirable for many reasons, officials understand that risk reduction strategies and detection of cases are imperative in creating a safe return to school. Furthermore, consequences of reclosing recently opened schools are substantial and impact teachers, parents, and ultimately educational experiences in children.

In this talk, I will present a compartmental model developed to explore scenarios under which reopening schools may be deemed safe and to evaluate mitigation strategies. Specifically, the question of differences in transmissibility will be discussed alongside a multiple cohort approach.

Zoom access

Meeting ID:
Zoom meeting starts at 3:30pm

### Wednesday, February 24

Algebra and Combinatorics Seminar

Topic: Invariants and properties of symbolic powers of edge and cover ideals

Joseph Skelton | Tulane University

Abstract: In this talk I will address several questions about symbolic powers of edge and cover ideals. The containment between ordinary and symbolic powers of edge ideals has been an active area of research for decades. As a result the resurgence number and Waldschmidt constant are of particular interest. The regularity of symbolic powers of edge ideals has been motivated by a conjecture of N.C. Minh which states that $\reg I(G)^{(s)} = \reg I(G)^s$ for any $s\in \NN$.

For cover ideals we are motivated by the results of Villlarreal showing that whiskering a graph results in a Cohen-Macaulay graph which, in turn, implies the cover ideal of the whiskered graph has linear resolution. Necessary and sufficient conditions on $S\subset V(G)$ cover ideal of the graph whiskered at $S$, $J(G\cup W(S))$ is Cohen-Macaulay. While symbolic powers of the cover ideal do not necessarily have linear resolution I will show necessary conditions on $S$ such that symbolic powers of $J(G\cup W(S))$ have componentwise linearity.

Zoom access
Time: 3:00pm

### Tuesday, February 23

Topic: A tutorial on Peirce's Graphical Logic

Alex Nisbet | Tulane University

Abstract: Charles S. Peirce was in important figure in the development of the algebra of logic'' in the second half of the 19th and early 20th century. In particular, he is known for his contributions in developing Boole's system, the logic of relatives, and quantification. Indeed, the algebraic notation he used is more-or-less what we use today, after some symbolic changes by Peano. Less well-known is Peirce's system of graphical logic, which he preferred to the algebraic notation. Here I will give a brief tutorial on the alpha and beta parts of this system. The alpha part corresponds to propositional logic and the beta to first order logic with equality.

Time: 5:00pm

Zoom ID: 954 0971 2190

Week of February  19 -  February 15

### Friday, February 19

Applied and Computational Mathematics

Topic: Convergence of unadjusted Hamiltonian Monte Carlo for mean-field models

Katharina Schuh | Hausdorff Center for Mathematics, University of Bonn

Abstract: In the talk, we consider the unadjusted Hamiltonian Monte Carlo algorithm applied to highdimensional probability distributions of mean-field type. We evolve dimension-free convergence and discretization error bounds. These bounds require the discretization step to be sufficiently small, but do not require strong convexity of either the unary or pairwise potential terms present in the mean-field model. To handle high dimensionality, we use a particlewise coupling that is contractive in a complementary particlewise metric. This talk is based on joint work with Nawaf Bou-Rabee.

Zoom access

Meeting ID:
Zoom meeting starts at 3:30pm

### Wednesday, February 17

Algebra and Combinatorics Seminar

Topic: A Survey of Classical Representation Theory

Mike Joyce | Tulane University

Abstract: Representation theory is a vast field which has applications in many other areas of mathematics, including algebra and combinatorics. This talk will review some of the classical theory of representations of groups and Lie algebras, with an emphasis thats lead to more modern aspects of representation theory, which will be addressed in a second talk.

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Time: 3:30pm

Week of February  12 -  February 8

### Friday, February 12

Applied and Computational Mathematics

Topic: Unravelling A Geometric Conspiracy

Michael Betancourt | Symplectomorphic, LLC

Abstract: The Hamiltonian Monte Carlo method has proven a powerful approach to efficiently exploring complex probability distributions.  That power, however, is something of a geometric conspiracy: a sequence of delicate mathematical machinations that ensure a means to explore distributions not just in theory but also in practice.  In this talk I will discuss the coincident geometrical properties that ensure the scalable performance of Hamiltonian Monte Carlo and present recent work developing new geometric theories that generalize each of these properties individually, providing a foundation for generalizing the method without compromising its performance."

The geometrical concepts get nontrivial towards the end but hopefully it will be sufficiently engaging for many!

Zoom access

Meeting ID:
Zoom meeting starts at 3:30pm

### Thursday, February 11

Colloquium

Topic: Class numbers of quadratic fields

Olivia Beckwith | University of Illinois Urbana-Champaign (Host: Lisa Fauci)

Abstract: Gauss was the first to count classes of binary quadratic forms with a fixed discriminant up to matrix equivalence. The number of equivalence classes, the class number, measures the obstruction to unique factorization into primes for quadratic number fields. Information about class numbers percolates into many branches of number theory, including the theory of L-functions via Dirichlet's class number formula, and elliptic curves in view of the work and conjecture of Birch and Swinnerton-Dyer. This talk will begin with a brief introduction to algebraic number theory and class numbers, as well as some of the important results in the history of their study. Then I will discuss some of my work in this area, which is about the divisibility properties of class numbers.

Zoom access: Contact mbrown2@math.tulane.edu
Time: 3:30pm

### Tuesday, February 9

Topic: Linearity Testing

Victor Bankston | Tulane University

Abstract: We will discuss techniques for deciding if a Boolean function is linear.

Zoom ID: 954 0971 2190
Time: 5:00pm
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Colloquium

Topic: Brownian Dynamics with Constraints

Brennan Sprinkle | New York University

Abstract: At the scale of a few micrometers, objects suspended in a fluid are subject to random kicks from collisions with the solvent molecules. This leads to a random motion of the suspended objects which must be reconciled with any geometric or mechanical constraints, like rigidity or inextensibility. After discussing numerical methods to simulate the Brownian dynamics of rigid bodies, I will primarily focus on simulation methods for inextensible filaments. Filaments at the cellular scale can take the form of beating flagella that propel sperm cells and bacteria; or they can tangle into the vast, interconnected networks that make up the cellular cytoskeleton. I’ll introduce a method where fibers are treated as a chain of beads and use it to interrogate experimental observations on magnetic filaments which can be made to swim using an applied field.  Motivated by this study, I’ll present ongoing work concerning a method more suited to fiber networks in which inextensible fiber motions are parametrized as curves on the unit sphere.

Zoom access: Contact mbrown2@math.tulane.edu
Time: 3:30pm

Special Colloquium

Topic: Edge Ideals of Random Graphs

Arindam Banerjee | (at Ramakrishna Mission Vivekananda Educational and Research Institute)

Abstract: The theory of edge ideal studies finite simple graphs from an algebraic perspective. It attaches an ideal with every finite simple graph and tries to interpret the combinatorics of the graph in terms of various algebraic invariants of that ideal. Philosophically speaking, one may ask what happens to those invariants on an average when one considers all possible graphs. The study of the edge ideals of Erdos-Renyi random graphs gives a nice mathematical framework to properly pose that philosophical question. In this talk we shall develop this framework and use that to discuss average behaviours of some important algebraic invariants. In particular we shall discuss a new result which shows that algebraic invariants Krull Dimension and  Castelnuovo-Mumford regularity (a measure of size and a measure of complexity respectively) satisfy some law of large number when number of vertices of the underlying Erdos-Renyi random graph goes to infinity.

Zoom access: Contact mbrown2@math.tulane.edu
Time:
10:30AM – 11:30AM

Week of February  5 -  February 1

### Thursday, February 4

Colloquium

Topic: The Batchelor spectrum and mixing in stochastic fluids

Samuel Punshon-Smith | Brown University (Scott McKinley)

Abstract: In 1959 George Batchelor predicted that a passively advected quantity in a fluid (like small temperature fluctuations or some chemical concentration), in a regime where the scalar dissipation is much lower than the fluid viscosity, should reach an equilibrium with an L2 spectral density proportional to 1/|k| over an appropriate inertial range, known as "the Batchelor spectrum". This prediction has since been observed experimentally and in various numerical experiments. However, despite strong evidence in its favor, rigorous derivations are only known in very special circumstances.

In this talk, I will consider the problem of a passive scalar undergoing advection-diffusion when the advecting velocity field belongs to a class of stochastic incompressible fluid motions, including models like the 2d incompressible stochastic Navier-Stokes equations (among a host of other stochastic fluid models in both 2d and 3d). I will discuss how a version of Batchelor's prediction is actually a general consequence of uniform-in-diffusivity exponential mixing properties of a fluid. Based on an argument inspired by techniques of Furstenberg for random dynamical systems, I will present a result deducing almost sure chaotic motion of the particle trajectories, known as Lagrangian chaos. I will then discuss how Lagrangian chaos can be used along with spectral theory and quantitative techniques from the ergodic theory of Markov processes to deduce almost sure, uniform-in-diffusivity exponential mixing, a powerful property that is not known to hold in the deterministic setting. This result shows that a version of Batchelor's prediction is indeed fairly robust and holds quite generally for a variety of suitably non-degenerate, stochastic incompressible fluid motions.

Zoom access: Contact mbrown2@math.tulane.edu
Time: 3:30pm

### Wednesday, February 3

Algebra and Combinatorics Seminar

Topic: The Least Generating Degree of Symbolic Powers and Ideal Containment Problem

Thai Nguyen | Tulane University

Abstract: What is the smallest degree of a homogeneous polynomial that vanishes to order m on a given finite set of points, or more generally on some algebraic variety in projective space? A classical result of Zariski and Nagata tells us the set of such polynomials is the m-th symbolic power of the defining ideal I of the variety. To bound the generating degree of the symbolic powers of I, we can study containment between symbolic powers and ordinary powers of I. Conversely, knowing bounds for generating degree can help us study containment. My talk will be an introduction to this subject. I will also present some results from our joint work with Sankhaneel Bisui, Eloísa Grifo and Tài Huy Hà.

Zoom access
Time: 3:00pm

### Tuesday, February 2

Topic: Living your best life with symbolic math software

Dana Ferranti | Tulane University

Abstract: I've only recently started using symbolic math software in my research and I wish I started earlier. This talk will focus on a few ways that one can use symbolic math software to make their workflow more efficient, as well as its limitations. For my examples I will be using the open source SymPy, however, the concepts will be applicable to other software.

Time: 5:00pm

Zoom ID
: 954 0971 2190

Colloquium

Topic: Rigidity of plane frameworks with forced symmetry

Daniel Bernstein | Fields Institute

Abstract: Rigidity theory asks and answers questions about how a given mechanical structure can deform. This area extends back into the nineteenth century with work of Cauchy and Maxwell, and continues to be an active area of research with a wide range of applications. I will begin my talk with a broad overview of this area. Then, I will narrow my focus onto symmetry-forced rigidity of plane frameworks to discuss a recent result. I will discuss the algebraic-geometric ideas involved in the proof, and how these same ideas can be used to address certain problems in matrix completion.

Zoom access: Contact mbrown2@math.tulane.edu
Time: 3:30pm

Week of January  29 -  January 25

### Friday, January 29

Applied and Computational Mathematics

Topic: DNA methylation-based aging biomarkers in health and disease

Mary Sehl | UCLA

Abstract: DNA methylation-based estimates of age are strongly correlated with chronologic age across many cell types and tissues. Importantly, these biologic aging estimates are accelerated in disease states, and predictive of both lifespan and healthspan. Recent evidence suggests that female breast tissue ages faster than other parts of the body in healthy women, based on the Horvath pan-tissue epigenetic clock. Estrogens are thought to contribute to breast cancer risk through cell cycling and accelerated breast aging. We hypothesize that epigenetic breast aging is driven by lifetime estrogen exposure. In this talk, we will review the development and key features of several epigenetic clocks including Horvath’s pan-tissue clock and the Hannum clock, as well as second generation clocks including the Phenotypic age, Grim age, and Skin and Blood age clocks. We will describe findings from a recent study examining associations between hormonal factors (including earlier age at menarche, and body mass index) and these epigenetic aging measures in healthy women. We will further describe additional applications of peripheral blood methylation age estimates to study biologic age acceleration in HIV-infected men pre- and post-initiation of antiretroviral therapy, and in early stage breast cancer survivors undergoing radiotherapy and chemotherapy.

Zoom access

Meeting ID:
Zoom meeting starts at 3:30pm

### Thursday, January 28

Colloquium

Topic:  Mathematically Modeling the Mechanisms Behind Intra-Droplet and Droplet Field Patterning in Phase Separated Systems

Dr. Kelsey Gasior | Florida State University

Abstract:

Zoom access: Contact mbrown2@math.tulane.edu
Time: 3:30pm

### Wednesday, January 27

Algebra and Combinatorics

Topic:  Stable Harbourne-Huneke containment and Chudnovsky's Conjecture

Sankhaneel Bisui | Tulane University

Abstract:

Zoom access:
Time: 3:30pm

### Monday, January 25

Colloquium

Topic:  Modeling and analysis of complex systems — with a basis in zebrafish patterns

Alexandria Volkening | Northwestern University

Abstract: Many natural and social phenomena involve individual agents coming together to create group dynamics, whether they are cells in a skin pattern, voters in an election, or pedestrians in a crowded room. Here I will focus on the specific example of pattern formation in zebrafish, which are named for the dark and light stripes that appear on their bodies and fins. Mutant zebrafish, on the other hand, feature different skin patterns, including spots and labyrinth curves. All these patterns form as the fish grow due to the interactions of tens of thousands of pigment cells. The longterm motivation for my work is to better link genes, cell behavior, and visible animal characteristics — I seek to identify the specific alterations to cell interactions that lead to mutant patterns. Toward this goal, I develop agent-based models to simulate pattern formation and make experimentally testable predictions. In this talk, I will overview my models and highlight future directions. Because agent-based models are not analytically tractable using traditional techniques, I will also discuss the topological methods that we have developed to quantitatively describe cell-based patterns, as well as the associated nonlocal continuum limits of my models.