## Seminars and Colloquia by Series

### Local and Optimal Transport Perspectives on Uncertainty Quantification

Series
Applied and Computational Mathematics Seminar
Time
Monday, November 22, 2021 - 14:00 for 1 hour (actually 50 minutes)
Location
https://bluejeans.com/457724603/4379
Speaker
Dr. Amir SagivColumbia

In many scientific areas, deterministic models (e.g., differential equations) use numerical parameters. In real-world settings, however, such parameters might be uncertain or noisy. A more comprehensive model should therefore provide a statistical description of the quantity of interest. Underlying this computational problem is a fundamental question - if two "similar" functions push-forward the same measure, would the new resulting measures be close, and if so, in what sense? We will first show how the probability density function (PDF) of the quantity of interest can be approximated, using spectral and local methods. We will then discuss the limitations of PDF approximation, and present an alternative viewpoint: through optimal transport theory, a Wasserstein-distance formulation of our problem yields a much simpler and widely applicable theory.

### Computer assisted proof of transverse homoclinic chaos - a look under the hood

Series
CDSNS Colloquium
Time
Friday, November 19, 2021 - 13:00 for 1 hour (actually 50 minutes)
Location
Skiles 005; streaming via Zoom available
Speaker
J.D. Mireles JamesFlorida Atlantic University

Please Note: Talk will be held in-person in Skiles 005 and streamed synchronously. Zoom link-- https://us06web.zoom.us/j/83392531099?pwd=UHh2MDFMcGErbzFtMHBZTmNZQXM0dz09

My goal is to present a computer assisted proof of a non-trivial theorem in nonlinear dynamics, in full detail.  My (quite biased) definition of non-trivial is that there should be some infinite dimensional complications.  However, since I want to go through all the details, I need these complications to be as simple as possible.  So, I'll consider the Henon map, and prove that some 1 dimensional stable and unstable manifolds attached to a hyperbolic fixed point intersect transversally.  By Smale's theorem, this implies the existence of chaotic motions.  Recall that one can prove the existence chaotic dynamics for the Henon map more or less by hand using topological methods.  Yet transverse intersection of the manifolds is a stronger statement, and moreover the method I'll discuss generalizes to much more sophisticated examples where pen-and-paper fail.

The idea of the proof is to develop a high order polynomial expansion of the stable/unstable manifolds of the fixed point, to prove an a-posteriori theorem about the convergence and truncation error bounds for this expansion, and to check the hypotheses of this theorem using the computer.  All of this relies on the parameterization method of Cabre, Fontich, and de la Llave, and on finite numerical calculations using interval arithmetic to manage the inevitable roundoff errors. Once global enough representations of the local invariant manifolds are obtained and equipped with mathematically rigorous error bounds, it is a finite dimensional problem to establish that the manifolds intersect transversally.

Series
Algebra Student Seminar
Time
Friday, November 19, 2021 - 10:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Jaewoo JungGeorgia Tech

Recall that, for a variety $X$ in a projective space $\mathbb{P}^d$, the $X$-rank of a point $p\in \mathbb{P}^d$ is the least number of points of $X$ whose span contains the point $p$. Studies about $X$-ranks include some well-known and important results about various tensor ranks. For example,

• the rank of tensors is the rank with respect to Segre varieties,
• the rank of symmetric tensors, i.e. Waring rank, is the rank with respect to Veronese embeddings, and
• the rank of anti-symmetric tensors is the rank with respect to Grassmannians in its Plücker embedding.

In this talk, we focus on ranks with respect to Veronese embeddings of a projective line $\mathbb{P}^1$. i.e. symmetric tensor ranks of binary forms. We will discuss how to associate points in $\mathbb{P}^d$ with binary forms and I will introduce apolarity for binary forms which gives an effective method to study Waring ranks of binary forms. We will discuss various ranks on the Veronese embedding and some results on the ranks.

### Intersection Theory on tropical manifolds

Series
Tropical Geometry Seminar
Time
Thursday, November 18, 2021 - 12:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Andreas Gross

In their seminal paper on combinatorial Hodge theory, Adiprasito, Huh, and Katz showed, among other things, that a very specific set of toric varieties has Chow rings that satisfy Poincaré duality, even though the varieties are not compact. In joint work with Farbod Shokrieh, we generalize this statement to all toric varieties whose fans are supported on a tropically smooth set. This has several consequences in tropical intersection theory; most notably it allows us to prove the long-suspected duality between tropical cycles and cocycles.

In my talk I will assume no prior knowledge of tropical intersection theory. I will define tropical cycles and cocycles explicitly and explain how they are connected to the intersection theory of toric varieties and the Chow rings of fans appearing in combinatorial Hodge theory. Finally, we will see how to use the duality statement mentioned above to define the tropical intersection product.

### TBA by Christina Frederick

Series
School of Mathematics Colloquium
Time
Thursday, November 18, 2021 - 11:00 for 1 hour (actually 50 minutes)
Location
ONLINE
Speaker
Christina Frederick

### Computational tropical geometry

Series
Tropical Geometry Seminar
Time
Thursday, November 18, 2021 - 10:30 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Anton LeykinGeorgia Tech

From a perspective of an applied algebraic geometer, I will address several use cases of algorithmic machinery that goes hand in hand with the language of tropical geometry. One of the examples originates in dynamical systems and may shed (tropical) light on a long-standing conjecture in celestial mechanics.

### Ranks of matrices and the algebra of forgetfulness

Series
Tropical Geometry Seminar
Time
Thursday, November 18, 2021 - 09:30 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Matt BakerGeorgia Tech

I will discuss a general framework for studying what can be said about the rank of a matrix A over a field K if we only know certain crude features of A. For example, what can we say about rank(A) if we only know which entries are zero and which are nonzero? Or if K = R, what if we only know the signs of the entries of A? Or K is a normed field and we only know the absolute values? Or K=C and we only know the arguments? There are many partial answers to questions like this scattered throughout the literature, and I will explain how at least some of these results can be unified through a theory of ranks of matrices over hyperfields. This is work in progress with Tianyi Zhang.

### An Alexander method for infinite-type surfaces

Series
Geometry Topology Student Seminar
Time
Wednesday, November 17, 2021 - 14:00 for 1 hour (actually 50 minutes)
Location
Online (via BlueJeans)
Speaker
Roberta ShapiroGeorgia Tech

Given a surface S, the Alexander method is a combinatorial tool used to determine whether two self-homeomorphisms of S are isotopic. This statement was formalized in the case of finite-type surfaces, which are surfaces with finitely generated fundamental groups. A version of the Alexander method was extended to infinite-type surfaces by Hernández-Morales-Valdez and Hernández-Hidber. We extend the remainder of the Alexander method to include infinite-type surfaces.

In this talk, we will talk about several applications of the Alexander method. Then, we will discuss a technique useful in proofs dealing with infinite-type surfaces and provide a "proof by example" of an infinite-type analogue of the Alexander method.

This will be practice for a future talk and comments and suggestions are appreciated.

### Data-driven mechanistic modeling for personalized oncology

Series
Mathematical Biology Seminar
Time
Wednesday, November 17, 2021 - 11:00 for 1 hour (actually 50 minutes)
Location
ONLINE
Speaker
Heiko EnderlingMoffitt Cancer Center

In close collaboration with experimentalists and clinicians, mathematical models that are parameterized with experimental and clinical data can help estimate patient-specific disease dynamics and treatment success. This positions us at the forefront of the advent of ‘virtual trials’ that predict personalized optimized treatment protocols. I will discuss a couple of different projects to demonstrate how to integrate calculus into clinical decision making. I will present a variety of mathematical model that can be calibrated from early treatment response dynamics to forecast responses to subsequent treatment. This may help us to identify patient candidates for treatment escalation when needed, and treatment de-escalation without jeopardizing outcomes.

### Irregular $\mathbf{d_n}$-Process is distinguishable from Uniform Random $\mathbf{d_n}$-graph

Series
Graph Theory Seminar
Time
Tuesday, November 16, 2021 - 15:45 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Erlang SuryaGeorgia Institute of Technology

For a graphic degree sequence $\mathbf{d_n}= (d_1 , . . . , d_n)$ of graphs with vertices $v_1 , . . . , v_n$, $\mathbf{d_n}$-process is the random graph process that inserts one edge at a time at random with the restriction that the degree of $v_i$ is at most $d_i$ . In 1999, N. Wormald asked whether the final graph of random $\mathbf{d_n}$-process is "similar" to the uniform random graph with degree sequence $\mathbf{d_n}$ when $\mathbf{d_n}=(d,\dots, d)$. We answer this question for the $\mathbf{d_n}$-process when the degree sequence $\mathbf{d_n}$ that is not close to being regular. We used the method of switching for stochastic processes; this allows us to track the edge statistics of the $\mathbf{d_n}$-process. Joint work with Mike Molloy and Lutz Warnke.