## Seminars and Colloquia Schedule

Monday, April 15, 2019 - 11:15 , Location: Skiles 005 , L. Serdukova , School of Mathematics, Georgia Institute of Technology , Organizer: Larissa Serdukova

Stability and bifurcation conditions for a vibroimpact motion in an inclined energy harvester with T-periodic forcing are determined analytically and numerically. This investigation provides a better understanding of impact velocity and its influence on energy harvesting efficiency and can be used to optimally design the device. The numerical and analytical results of periodic motions are in excellent agreement. The stability conditions are developed in non-dimensional parameter space through two basic nonlinear maps based on switching manifolds that correspond to impacts with the top and bottom membranes of the energy harvesting device. The range for stable simple T-periodic behavior is reduced with increasing angle of incline β, since the influence of gravity increases the asymmetry of dynamics following impacts at the bottom and top. These asymmetric T-periodic solutions lose stability to period doubling solutions for β ≥ 0, which appear through increased asymmetry. The period doubling, symmetric and asymmetric periodic motion are illustrated by bifurcation diagrams, phase portraits and velocity time series.

Monday, April 15, 2019 - 12:45 , Location: Skiles 006 , , Boston College , , Organizer: JungHwan Park

I will give a brief survey of concordance in high-dimensional knot theory and how slice results have classically been obtained in this setting with the aid of surgery theory. Time permitting, I will then discuss an example of how some non-abelian slice obstructions come into the picture for 1-knots, as intuition for the seminar talk about L^2 invariants.

Monday, April 15, 2019 - 12:50 , Location: Skiles 005 , , Yale University , , Organizer: Yoav Len

Tropical geometry provides a new set of purely combinatorial tools, which has been used to approach classical problems. In tropical geometry most algebraic computations are done on the classical side - using the algebra of the original variety. The theory developed so far has explored the geometric aspect of tropical varieties as opposed to the underlying (semiring) algebra and there are still many commutative algebra tools and notions without a tropical analogue. In the recent years, there has been a lot of effort dedicated to developing the necessary tools for commutative algebra using different frameworks, among which prime congruences, tropical ideals, tropical schemes. These approaches allows for the exploration of the  properties of tropicalized spaces without tying them up to the original varieties and working with geometric structures inherently defined in characteristic one (that is, additively idempotent) semifields. In this talk we explore the relationship between tropical ideals and congruences to conclude that the variety of a prime (tropical) ideal is either empty or consists of a single point. This is joint work with D. Joó.

Monday, April 15, 2019 - 13:55 , Location: Skiles 005 , , New York University , , Organizer: Wenjing Liao

Information retrieval from graphs plays an increasingly important role in data science and machine learning. This talk focuses on two such examples. The first one concerns the graph cuts problem: how to find the optimal k-way graph cuts given an adjacency matrix. We present a convex relaxation of ratio cut and normalized cut, which gives rise to a rigorous theoretical analysis of graph cuts. We derive deterministic bounds of finding the optimal graph cuts via a spectral proximity condition which naturally depends on the intra-cluster and inter-cluster connectivity. Moreover, our theory provides theoretic guarantees for spectral clustering and community detection under stochastic block model. The second example is about the landscape of a nonconvex cost function arising from group synchronization and matrix completion. This function also appears as the energy function of coupled oscillators on networks. We study how the landscape of this function is related to the underlying network topologies. We prove that the optimization landscape has no spurious local minima if the underlying network is a deterministic dense graph or an Erdos-Renyi random graph. The results find applications in signal processing and dynamical systems on networks.

Monday, April 15, 2019 - 14:00 , Location: Skiles 006 , , Boston College , , Organizer: JungHwan Park

The question of which high-dimensional knots are slice was entirely solved by Kervaire and Levine. Compared to this, the question of which knots are doubly slice in high-dimensions is still a largely open problem. Ruberman proved that in every dimension, some version of the Casson-Gordon invariants can be applied to obtain algebraically doubly slice knots that are not doubly slice. I will show how to use L^2 signatures to recover the result of Ruberman for (4k-3)-dimensional knots, and discuss how the derived series of the knot group might be used to organise the high-dimensional doubly slice problem.

Tuesday, April 16, 2019 - 12:00 , Location: Skiles 005 , Kathlén Kohn , ICERM and University of Oslo , , Organizer: Greg Blekherman
Wachspress defined barycentric coordinates on polygons in 1975. Warren generalized his construction to higher dimensional polytopes in 1996. In contrast to the classical case of simplices, barycentric coordinates on other polytopes are not unique. So the coordinates defined by Warren are now commonly known as Wachspress coordinates. They are used in a variety of applications, such as geometric modeling.

We connect the constructions by Warren and Wachspress by proving the conjecture that there is a unique polynomial of minimal degree which vanishes on the non-faces of a simple polytope. This is the adjoint polynomial introduced by Warren. Our formulation is the natural generalization of Wachspress' original idea.

The algebraic geometry of the map defined by the Wachspress coordinates was studied in the case of polygons by Irving and Schenk in 2014. We extend their results to higher dimensional polytopes. In particular, we show that the image of this Wachspress map is the projection from the image of the adjoint. For three-dimensional polytopes, we show that their adjoints are adjoints of K3- or elliptic surfaces. This talk is based on joint works with Kristian Ranestad, Boris Shapiro and Bernd Sturmfels.

Series: PDE Seminar
Tuesday, April 16, 2019 - 15:00 , Location: Skiles 006 , Li Chen , University of Mannheim , , Organizer: Ronghua Pan

I this talk I will summerize some of our contributions in the analysis of parabolic elliptic Keller-Segel system, a typical model in chemotaxis. For the case of linear diffusion, after introducing the critical mass in two dimension, I will show our result for blow-up conditions for higher dimension. The second part of the talk is concentrated in the critical exponent for Keller-Segel system with porus media type diffusion. In the end, motivated from the result on nonlocal Fisher-KPP equation, we show that the nonlocal reaction will also help in preventing the blow-up of the solutions.

Wednesday, April 17, 2019 - 10:00 , Location: Skiles 006 , Haoyan Zhai , Georgia Tech , , Organizer: Haoyan Zhai

Optimal transport is a thoroughly studied field in mathematics and introduces the concept of Wasserstein distance, which has been widely used in various applications in computational mathematics, machine learning as well as many areas in engineering. Meanwhile, control theory and path planning is an active branch in mathematics and robotics, focusing on algorithms that calculates feasible or optimal paths for robotic systems. In this defense, we use the properties of the gradient flows in Wasserstein metric to design algorithms to handle different types of path planning and control problems as well as the K-means problems defined on graphs.

Wednesday, April 17, 2019 - 12:05 , Location: Skiles 006 , Christian Houdré , Georgia Tech , Organizer: Trevor Gunn

I will briefly present our Stochastics Group and its main interests, and will continue with some of the problems I have worked on in recent years.

Wednesday, April 17, 2019 - 14:00 , Location: Skiles 006 , Sudipta Kolay , Georgia Tech , Organizer: Sudipta Kolay

We will see some instances of swindles in mathematics, primarily focusing on some in geometric topology due to Barry Mazur.

Wednesday, April 17, 2019 - 15:00 , Location: Skiles 006 , , Georgia Tech , , Organizer: Galyna Livshyts

We discuss the asymptotic value of the maximal perimeter of a convex set in an n-dimensional space with respect to certain classes of measures. Firstly, we derive a lower bound for this quantity for a large class of probability distributions; the lower bound depends on the moments only. This lower bound is sharp in the case of the Gaussian measure (as was shown by Nazarov in 2001), and, more generally, in the case of rotation invariant log-concave measures (as was shown by myself in 2014). We discuss another class of measures for which this bound is sharp. For isotropic log-concave measures, the value of the lower bound is at least n^{1/8}.

In addition, we show a uniform upper bound of Cn||f||^{1/n}_{\infty} for all log-concave measures in a special position, which is attained for the uniform distribution on the cube. We further estimate the maximal perimeter of isotropic log-concave measures by n^2.

Wednesday, April 17, 2019 - 16:30 , Location: Skiles 006 , Michail Sarantis , Georgia Tech , Organizer: Xingxing Yu

The well known Erdos-Hajnal Conjecture states that every graph has the Erdos-Hajnal (EH) property. That is, for every $H$, there exists a $c=c(H)>0$ such that every graph $G$ with no induced copy of $H$ has the property $hom(G):=max\{\alpha(G),\omega(G)\}\geq |V(G)|^{c}$. Let $H,J$ be subdivisions of caterpillar graphs. Liebenau, Pilipczuk, Seymour and Spirkl proved that the EH property holds if we forbid both $H$ and $\overline{J}.$ We will discuss the proof of this result.

Thursday, April 18, 2019 - 15:00 , Location: Skiles 005 , David Galvin , University of Notre Dam , Organizer: Xingxing Yu

To
any finite real sequence, we can associate a permutation $\pi$, via:
$\pi(k)$ is the index of the $k$th smallest element of the sequence.
This association was introduced in a 1987 paper
of Alavi, Malde, Schwenk and Erd\H{o}s, where they used it to study the
possible patterns of rises and falls that can occur in the matching
sequence of a graph (the sequence whose $k$th term is the number of
matchings of size $k$), and in the independent set
sequence.

The
main result of their paper was that {\em every} permutation can arise
as the independent set permutation'' of some graph. They left open the
following extremal question: for each $n$, what is
the smallest order $m$ such that every permutation of $[n]$ can be
realized as the independent set permutation of some graph of order at
most $m$?

We
answer this question. We also improve Alavi et al.'s upper bound on the
number of permutations that can be realized as the matching permutation
of some graph. There are still many open questions
in this area.

This is joint work with T. Ball, K. Hyry and K. Weingartner, all at Notre Dame.

Thursday, April 18, 2019 - 15:05 , Location: Skiles 006 , Nizar Demni , University of Marseille , Organizer: Christian Houdre
Friday, April 19, 2019 - 12:00 , Location: Skiles 006 , Marc Härkönen , Georgia Tech ,
Friday, April 19, 2019 - 14:00 , Location: Skiles 006 , Arash Yavari and Fabio Sozio, School of Civil and Environmental Engineering , Georgia Tech , Organizer: Igor Belegradek

We formulate a geometric nonlinear theory of the mechanics of accretion. In this theory the material manifold of an accreting body is represented by a time-dependent Riemannian manifold with a time-independent metric that at each point depends on the state of deformation at that point at its time of attachment to the body, and on the way the new material isadded to the body. We study the incompatibilities induced by accretion through the analysis of the material metric and its curvature in relation to the foliated structure of the accreted body. Balance laws are discussed and the initial-boundary value problem of accretion is formulated. The particular cases where the growth surface is either fixed or traction-free are studied and some analytical results are provided. We numerically solve several accretion problems and calculate the residual stresses in nonlinear elastic bodies induced from accretion.

Friday, April 19, 2019 - 16:00 , Location: Skiles 005 , Pavel Svetlichnyy , School of Physics, GaTeach , , Organizer: Federico Bonetto

I will talk about a conjecture that in Gibbs states of one-dimensional spin chains with short-ranged gapped Hamiltonians the quantum conditional mutual information (QCMI) between the parts of the chain decays exponentially with the length of separation between said parts. The smallness of QCMI enables efficient representation of these states as tensor networks, which allows their efficient construction and fast computation of global quantities, such as entropy. I will present the known partial results on the way of proving of the conjecture and discuss the probable approaches to the proof and the obstacles that are encountered.