# Difference between revisions of "Applied Algebra Seminar Fall 2022"

When: 1pm Central, Thursdays

Where: Van Vleck 901 Hall or Virtual: Zoom link (The passcode: Bucky )

Email List: subscribe to our google group or email JIR to add you.

Contacts: Jose Israel Rodriguez (Lead), Shamgar Gurevich

How: Virtual talks are usually 40-45 minutes long with informal discussion after; in-person talks are 40-50 minutes long.

Inclusiveness: We welcome participants with applied algebra interests from other institutions and departments. We have social time 5-10 minutes before the talk starts.

## Fall 2022 Schedule

• As of 8/3/22, dates may be pushed up or back a week to accommodate a speaker when necessary.
date Location speaker title host(s)
August 18 Social Time ---
September 8 VV 901 Hanbaek Lyu Phase transition in contingency tables with two linear margins (Local)
September 22 VV 901 Aravind Baskar Optimal synthesis of robotic mechanisms using homotopy continuation Rodriguez
October 6
October 20
November 3 VV 901 Zvi Rosen TBD Rodriguez
November 10

## Spring 2022 Schedule

date Location speaker title host(s)
Aug 18 Social Time ---
February 10 Zoom Diego Cifuentes (Georgia Tech) Polynomial time guarantees for the Burer-Monteiro method
February 24 Zoom Ola Sobieska (Wisconsin) Identifiability of Linear Compartmental Models: The Impact of Removing Leaks and Edges
March 11 (Friday, 10am) Zoom Chung Pang Mok (Soochow) Pseudorandom Vectors Generation Using Elliptic Curves And Applications to Wiener Processes
March 24 Zoom Daniel Pimentel Alarcon (Wisconsin) Intersecting Schubert Varieties for Subspace Clustering
April 14 Zoom Abeer Al Ahmadieh (Washington) Determinantal Representations and the Image of the Principal Minor Map
April 28 Zoom Kathryn Heal (Harvard) A Lighting-Invariant Point Processor for Shape from Shading

## Fall 2021 Schedule

date speaker title host(s)
September 16 Jeroen Zuiddam (University of Amsterdam) Geometric rank of tensors and applications Virtual
October 7 Timo de Wolff (TU Braunschweig) Certificates of Nonnegativity and Maximal Mediated Sets Virtual
October 21 Jesús A. De Loera (UC Davis) A Combinatorial Geometry View of Inference and Learning Problems Virtual
November 4 Kevin Shu (Georgia Tech) Nonnegative Quadratics over Stanley Reisner Varieties Virtual
November 18 Angélica Torres (KTH Stockholm) The line multi-view variety in computer vision Virtual
December 2 Papri Dey (Missouri) Hyperbolic Polynomials, and Helton-Vinnikov curves Virtual

## Spring 2021 Schedule

date speaker title host(s)
February 25 Greg Blekherman (Georgia Tech) Locally Positive Semidefinite Matrices Virtual
March 25 James Saunderson (Monash University) Lifting For Simplicity: Concise Descriptions of Convex Sets Virtual
April 22 (At the new 11am time!) Hamza Fawzi (Cambridge) Semidefinite representations, and the set of separable states Virtual
May 13 (At the new 11am time!) Anna Seigal (University of Oxford) Invariant Theory for Maximum Likelihood Estimation Virtual

## Spring 2020 Schedule

date speaker title host(s)
February 20 Carla Michini (UW Madison) Short simplex paths in lattice polytopes Local
March 5 Alisha Zachariah (UW Madison) Efficient Estimation of a Sparse Delay-Doopler Channel Local
March 19 Spring Break
March 26 (Seminar on Hiatus because of Covid-19)

## Abstracts 2022 Fall

### Hanbaek Lyu

##### Phase transition in contingency tables with two linear margins

Contingency tables are matrices of nonnegative integer entries with prescribed row sums and columns sums (margins). They are fundamental objects in statistics for studying dependence structure between two or more variables, and they also correspond to bipartite multi-graphs with given degrees and play an important role in combinatorics and graph theory. Both counting the number of contingency tables and sampling one uniformly at random are fundamental problems concerning contingency tables with many connections and applications to other fields (e.g., testing hypothesis on co-occurrence of species in ecology).

A historic guiding principle to these problems is the independence heuristic, which was introduced by I. J. Good as far back as in 1950. The heuristic states that the constraints for the rows and columns of the table are asymptotically independent as the size of the table grows to infinity. This yields a simple yet surprisingly accurate formula for the count of contingency tables as well as a sampling heuristic using the hypergeometric (or Fisher-Yates) distribution. In this talk, we will discuss that the independence heuristic captures only one part of the picture. Specifically, for n by n contingency tables with two linear margins $BCn$ and $Cn$, we establish that contingency tables exhibit a sharp phase transition at $B = B_{c} = 1+\sqrt{1+1/C}. This yields that the independence heuristic may capture the structure of contingency tables well for near-homogeneous margins (when$B<B_{c}$), but in if the margins are not quite homogeneous ($B>B_{c}$) positive correlation between rows and columns may emerge and the heuristic fails dramatically. ## Earlier Abstracts ### Kathryn Heal A Lighting-Invariant Point Processor for Shape from Shading Under the conventional diffuse shading model with unknown directional lighting, the set of quadratic surface shapes that are consistent with the spatial derivatives of intensity at a single image point is a two-dimensional algebraic variety embedded in the five-dimensional space of quadratic shapes. In this talk will describe the geometry of this variety, and introduce a concise feedforward model that computes an explicit, differentiable approximation of the variety from the intensity and its derivatives at any single image point. The result is a parallelizable processor that operates at each image point and produces a lighting-invariant descriptor of the continuous set of compatible surface shapes at the point. I will describe two applications of this processor: two-shot uncalibrated photometric stereo and quadratic-surface shape from shading. This talk will touch on both theory and application. ### Abeer Al Ahmadieh Determinantal Representations and the Image of the Principal Minor Map The principal minor map takes an n by n square matrix to the length$2^n$-vector of its principal minors. A basic question is to give necessary and sufficient conditions that characterize the image of various spaces of matrices under this map. In this talk I will describe the image of the space of complex matrices using a characterization of determinantal representations of multiaffine polynomials, based on the factorization of their Rayleigh differences. Using these techniques I will give equations and inequalities characterizing the images of the spaces of real and complex symmetric and Hermitian matrices. For complex symmetric matrices this recovers a result of Oeding from 2011. This is based on a joint work with Cynthia Vinzant. ### Daniel Pimentel Alarcon Intersecting Schubert Varieties for Subspace Clustering Subspace clustering aims to assign a collection of data points to the smallest possible union of subspaces. This is particularly challenging when data is missing. In this talk I will describe a new approach to overcome missing data. The main idea is to search for subspaces in the Grassmannian that simultaneously fit multiple incomplete-data points. This boils down to identifying the intersection of the Schubert varieties defined by the observed data, which is easier said than done. I will tell you what we have achieved so far, and the remaining challenges. ### Chung Pang Mok Pseudorandom Vectors Generation Using Elliptic Curves And Applications to Wiener Processes Using the arithmetic of elliptic curves over finite fields, we present an algorithm for the efficient generation of sequence of uniform pseudorandom vectors in high dimension with long period, that simulates sample sequence of a sequence of independent identically distributed random variables, with values in the hypercube$[0,1]^d$with uniform distribution. As an application, we obtain, in the discrete time simulation, an efficient algorithm to simulate, uniformly distributed sample path sequence of a sequence of independent standard Wiener processes. ### Ola Sobieska Identifiability of Linear Compartmental Models: The Impact of Removing Leaks and Edges In biology, pharmacokinetics, and various other fields, linear compartmental models are used to represent the transfer of substances between compartments in a system, e.g. the interchange of a drug between organs in the body. Such a model is identifiable if we can recover the flow parameters from generic input and output data. In this talk, I analyze changes to identifiability resulting from deleting a pathway between two compartments or from adding or deleting an internal leak. In particular, I will present some criteria for identifiability dependant on the combinatorics of the parent model, as well as a restatement and partial answer to a conjecture of Gross, Harrington, Meshkat, and Shu. This is joint work with Patrick Chan, Katherine Johnston, Anne Shiu, and Clare Spinner. ### Diego Cifuentes Polynomial time guarantees for the Burer-Monteiro method The Burer-Monteiro method is one of the most widely used techniques for solving large-scale semidefinite programs (SDP). The basic idea is to solve a nonconvex program in Y, where Y is an n×p matrix such that X = YYT. In this paper, we show that this method can solve SDPs in polynomial time in a smoothed analysis setting. More precisely, we consider an SDP whose domain satisfies some compactness and smoothness assumptions, and slightly perturb the cost matrix and the constraints. We show that if p ≳ √(2+2η)m, where m is the number of constraints and η>0 is any fixed constant, then the Burer-Monteiro method can solve SDPs to any desired accuracy in polynomial time, in the setting of smooth analysis. Our bound on p approaches the celebrated Barvinok-Pataki bound in the limit as η goes to zero, beneath which it is known that the nonconvex program can be suboptimal. ### Papri Dey Hyperbolic Polynomials, and Helton-Vinnikov curves In this talk, I shall give a panoramic view of my research work. I shall introduce the notion of hyperbolic polynomials and discuss an algebraic method to test hyperbolicity of a multivariate polynomial w.r.t some fixed point via sum-of-squares relaxation, proposed in my research work. An important class of hyperbolic polynomials are definite determinantal polynomials. Helton-Vinnikov curves in the projective plane are cut-out by hyperbolic polynomials in three variables. This leads to the computational problem of explicitly producing a symmetric positive definite linear determinantal representation for a given curve. I shall focus on two approaches to this problem proposed in my research work: an algebraic approach via solving polynomial equations, and a geometric-combinatorial approach via scalar product representations of the coefficients and its connection with polytope membership problem. The algorithms to solve determinantal representation problems are implemented in Macaulay2 as a software package DeterminantalRepresentations. m2. ### Angélica Torres The line multi-view variety in computer vision Given an arrangement of cameras, the line multi-view variety is the Zariski closure of the image of world lines in the cameras. This is a similar definition to the multi-view variety studied extensively in computer vision. The goal of this talk is to present a description of the ideal of the line multi-view variety, and discuss some geometric properties of this variety. This is joint work with Felix Rydell, Elima Shehu, Paul Breiding, and Kathlén Kohn. ### Kevin Shu Nonnegative Quadratics over Stanley Reisner Varieties Nonnegative polynomials are of fundamental interest in the field of real algebraic geometry. We will discuss the convex geometry of nonnegative polynomials over an interesting class of algebraic varieties known as Stanley-Reisner varieties. Stanley-Reisner varieties are fairly simple, as they are unions of coordinate planes, but they have surprising connections to algebraic topology that we can use to obtain a detailed understanding of the convex geometry of these nonnegative quadratics. We will be particularly interested in the extreme nonnegative quadratic forms over such varieties. Though this will not be emphasized in the talk, this subject also has some direct applications to optimization theory. ### Jesús A. De Loera A Combinatorial Geometry View of Inference and Learning Problems In statistical inference we wish to learn the properties or parameters of a distribution through sufficiently many samples. A famous example is logistic regression, a popular non-linear model in multivariate statistics and supervised learning. Users often rely on optimizing of maximum likelihood estimation, but how much training data do we need, as a function of the dimension of the covariates of the data, before we expect an MLE to exist with high probability? Similarly, for unsupervised learning and non-parametric statistics, one wishes to uncover the shape and patterns from samples of a measure or measures. We use only the intrinsic geometry and topology of the sample. A famous example of this type of method is the$k$-means clustering algorithm. A fascinating challenge is to explain the variability of behavior of$k\$-means algorithms with distinct random initializations and the shapes of the clusters.

In this talk we present new stochastic combinatorial theorems, that give bounds on the probability of existence of maximum likelihood estimators in multinomial logistic regression and also relate to the variability of clustering initializations. Along the way we will see fascinating connections to the coupon collector problem, topological data analysis, and to the computation of Tukey centerpoints of data clouds (a high-dimensional generalization of median). This is joint work with T. Hogan, R. D. Oliveros, E. Jaramillo-Rodriguez, and A. Torres-Hernandez.

### Timo de Wolff

Certificates of Nonnegativity and Maximal Mediated Sets

In science and engineering, we regularly face polynomial optimization problems, that is: minimize a real, multivariate polynomial under polynomial constraints. Solving these problems is essentially equivalent to certifying of nonnegativity of real polynomials -- a key problem in real algebraic geometry since the 19th century. Since this problem is notoriously hard to solve, one is interested in certificates that imply nonnegativity and are easier to check than nonnegativity itself. In particular, a polynomial is nonnegative if it is a sums of squares (SOS) of other polynomials. In 2014, Iliman and I introduced a new certificate of nonnegativity based on sums of nonnegative circuit polynomials (SONC), which I have developed further since then both in theory and practice joint with different coauthors.

A canonical question is: When is a nonnegative circuit polynomial also a sums of squares. It turns out that this question is purely combinatorial and is governed by sets of particular (integral) lattice points called maximal mediated sets, which where first introduced by Reznick. In this talk, I will give a brief introduction to nonnegativity, SOS, and SONC, and then discuss recent results on maximal mediated sets.

### Jeroen Zuiddam

Geometric rank of tensors and applications

There are several important problems in computational complexity and extremal combinatorics that can naturally be phrased in terms of tensors. This includes the matrix multiplication problem, the cap set problem and the sunflower problem. We present a new geometric method in this area, called Geometric Rank (Kopparty–Moshkovitz–Zuiddam, 2020). Geometric Rank is a natural extension of matrix rank to tensors. As an application, we use Geometric Rank to solve a problem of Strassen about matrix multiplication tensors.

### Anna Seigal

Invariant Theory for Maximum Likelihood Estimation

Maximum likelihood estimation is an optimization problem over a statistical model, to obtain the parameters that best fit observed data. I will focus on two settings: log-linear models and Gaussian group models. I will describe connections between maximum likelihood estimation and notions of stability from invariant theory. This talk is based on joint work with Carlos Améndola, Kathlén Kohn and Philipp Reichenbach.

### Hamza Fawzi

Semidefinite representations, and the set of separable states

The convex set of separable states plays a fundamental role in quantum information theory as it corresponds to the set of non-entangled states. In this talk I will discuss the question of semidefinite programming representations for this convex set. Using connections with nonnegative polynomials and sums of squares I will show that except in the low-dimensions, this semialgebraic set has no semidefinite representation. As a consequence this set furnishes a counter-example to the Helton-Nie conjecture. The proof I will present is elementary and relies only on basic results about semialgebraic sets and functions.

### James Saunderson

Lifting For Simplicity: Concise Descriptions of Convex Sets

This talk will give a selective overview of the theory and applications of lifts of convex sets. A lift of a convex set is a higher-dimensional convex set that projects onto the original set. Many interesting convex sets have lifts that are dramatically simpler to describe than the original set. Finding such simple lifts has significant algorithmic implications, particularly for associated optimization problems. We will consider both the classical case of polyhedral lifts, which are described by linear inequalities, as well as spectrahedral lifts, which are defined by linear matrix inequalities. The talk will include discussion of ways to construct lifts, ways to find obstructions to the existence of lifts, and a number of interesting examples from a variety of mathematical contexts. (Based on joint work with H. Fawzi, J. Gouveia, P. Parrilo, and R. Thomas).

### Greg Blekherman

Locally Positive Semidefinite Matrices

The cone of positive semidefinite matrices plays a prominent role in optimization, and many hard computational problems have well-performing semidefinite relaxations. In practice, enforcing the constraint that a large matrix is positive semidefinite can be expensive. We introduce the cone of k-locally posiitive semidefinite matrices, which consists of matrices all of whose k by k principal submatrices are positive semidefinite. We consider the distance between the cones of positive and locally positive semidefinite matrices, and possible eigenvalues of locally positive semidefinite matrices. Hyperbolic polynomials play a role in some of the proofs. Joint work with Santanu Dey, Marco Molinaro, Kevin Shu and Shengding Sun.

### Carla Michini

Short simplex paths in lattice polytopes

We consider the problem of optimizing a linear function over a lattice polytope P contained in [0,k]^n and defined via m linear inequalities. We design a simplex algorithm that, given an initial vertex, reaches an optimal vertex by tracing a path along the edges of P of length at most O(n^6 k log k). The length of this path is independent on m and is the best possible up to a polynomial function, since it is only polynomially far from the worst case diameter. The number of arithmetic operations needed to compute the next vertex in the path is polynomial in n, m and log k. If k is polynomially bounded by n and m, the algorithm runs in strongly polynomial time. This is a joint work with Alberto Del Pia.

### Alisha Zachariah

Efficiently Estimating a Sparse Delay-Doppler Channel

Multiple wireless sensing tasks, e.g., radar detection for driver safety, involve estimating the ”channel” or relationship between signal transmitted and received. In this talk, I will focus on a certain type of channel known as the delay-doppler channel. This channel model starts to be applicable in high frequency carrier settings, which are increasingly common with recent developments in mmWave technology. Moreover, in this setting, both the channel model and existing technologies are amenable to working with signals of large bandwidth, and using such signals is a standard approach to achieving high resolution channel estimation. However, when high resolution is desirable, this approach creates a tension with the desire for efficiency because, in particular, it immediately implies that the signals in play live in a space of very high dimension N (e.g., ~10^6 in some applications), as per the Shannon-Nyquist sampling theorem.

To address this, I will propose a randomized algorithm for channel estimation in the k-sparse setting (e.g., k objects in radar detection), with sampling and space complexity both on the order of k(log N)^2, and arithmetic complexity on the order of k(log N)^3+k^2, for N sufficiently large.

While this algorithm seems to be extremely efficient -- to the best of our knowledge, the first of this nature in terms of complexity -- it is just a simple combination of three ingredients, two of which are well-known and widely used, namely digital chirp signals and discrete Gaussian filter functions, and the third being recent developments in Sparse Fast Fourier Transform algorithms.

## Related events to note

date event/title location/speaker info
Postponed Applied Algebra Days 4 - Tensors Several talks on tensors
1:30pm, 2nd Thursday of the month Informal Seminar: Algebra in Statistics and Computation Virtual
3:30pm SIAM Student Chapter Virtual
10am, 2nd Tuesday of the month SIAM SAGA Virtual: Recordings Registration needed once.
10am, Most Tuesdays Nonlinear algebra seminar online Virtual: Recordings Registration required once
Biweekly Mondays Algebraic Statistics Online Seminar (ASOS) Virtual: Recordings Mailing list sign-up for Zoom-links
January 26th-29th, 2021 Sanya Workshop on Algebraic Geometry and Machine Learning Virtual: Recordings
July 29-30, 2021 Real algebraic geometry and convex geometry Conference TBD: TU Braunschweig, Germany or Online