This seminar features talks on interdisciplinary research which involves applications of fields related to algebra, geometry, topology, and combinatorics to fields like statistics, optimization, computer science, electrical engineering, biology, physics, and other sciences. One of the main goals of the seminar is to connect people from pure mathematics with people from applied fields.

**Contact:** Elina Robeva, erobeva@mit.edu; Elisa Perrone , eperrone@mit.edu

**Mailing list:** If you would like to be added to the mailing list, please contact one of the organizers above .

In the fall of 2018, we are going to hold an *Applied Algebra Day*. Here are the logistics.

** Date:** Saturday Novermber 17 2018

** Time:** 9:30AM - 5PM

** Location:** E17-304 (access from 50 Ames Street)

** Titles and abstracts **

** Speakers and program: **

- 9:30am
*Solving elliptic PDEs on curves and surfaces*Jonathan Hauenstein, University of Notre Dame - 10am Oriented Gaussoids Thomas Kahle, OvGU Magdeburg
- 10:30am
*-- coffee break --* - 11am
*The degree of the central curve of semidefinite programming*Elias Tsigaridas, Inria Paris - 11:30am
*A minimal approach to minimal problems in 3D reconstruction*Anton Leykin, GATech - 12pm
*Distinguishing and Inferring Phylogenetic Networks*Elizabeth Gross, U Hawaii - 12:30pm
*-- lunch break --* - 2pm
*Rank-metric codes and q-polymatroids*Elisa Gorla, U Neuchatel - 2:30pm
*Positively Hyperbolic Varieties and Tropical Geometry*Josephine Yu, GATech - 3pm
*Tell the truth and never lose with tropical geometry*Ngoc Tran, UT Austin - 3:30pm
*-- coffee break --* - 4pm
*Tropical Geometry of Deep Neural Networks*Gregory Naitzat, UChicago - 4:30pm
*Fast Computations of Monomial Ideal Invariants Using Constraint Integer Programming*Lily Silverstein, UC Davis - 5pm
*Robust Estimation of High-Dimensional Graphical Models under Total Positivity*Yuhao Wang, MIT

April 23, 2018, 1:30-2:30pm: **Jesús De Loera** University of California, Davis

Title: Random Monomial Ideals

Abstract: Randomness is a key algorithmic tool in algebra. In this
talk I will discuss our recent work describing the random behavior of
monomial ideals. Our work is a natural generalization of classical work on
random graphs and random simplicial complexes. Under this model we outlined some
basic properties of random monomial ideals. For example, I will present theorems
about the probability distributions, expectations and thresholds for events
of monomial ideals with given Krull dimension, Hilbert function, and
the average behavior of minimal-free resolutions.

April 23, 2018, 2:45-3:45pm: **Anna Seigal** University of California, Berkeley

Title: Ranks and Symmetric Ranks of Cubic Surfaces

Abstract: We study cubic surfaces as symmetric tensors of format 4 x 4 x 4. We consider the non-symmetric rank and the symmetric rank of cubic surfaces, and show that the two notions coincide over the complex numbers.
As part of our analysis, we give a test for symmetric rank given by the non-vanishing of certain discriminants. The corresponding algebraic problem concerns border ranks. We show that the non-symmetric border rank coincides with the symmetric border rank for cubic surfaces, and obtain minimal ideal generators for the symmetric analogue to the secant variety from the salmon conjecture. We also discuss how the results extend to tensors of larger sizes.

September 19, 2017: **Carlos Améndola** TU Berlin

Title: Discrete Gaussian Distributions Via Theta Functions

Abstract: Maximum entropy probability distributions are important for information theory and relate directly to exponential families in statistics. Having the property of maximizing entropy can be used to define a discrete analogue of the classical continuous Gaussian distribution. We present a parametrization of such a density using the Riemann Theta function, use it to derive fundamental properties which include computing its characteristic function, and exhibit connections to the study of abelian varieties. This is ongoing joint work with Daniele Agostini (HU Berlin).

October 3, 2017: **Kaie Kubjas** MIT

Title: Geometry and Maximum Likelihood Estimation of the Binary Latent Class Model

Abstract: The Expectation-Maximization algorithm is commonly used for the maximum likelihood estimation of the latent class models. However, it does not guarantee finding the global optimum. We study the geometry of the binary latent class model and in the case of three observed binary variables, apply it to obtain closed formulas for the MLE. The second approach is taken through the study of the EM fixed point ideal. In the case of a binary latent class model with a binary or ternary hidden variable, we show how to get the boundary stratification of the model by decomposing the EM fixed point ideal. This talk is based on the joint work with Elizabeth Allman, Hector Banos Cervantes, Robin Evans, Serkan Hosten, Daniel Lemke, John Rhodes and Piotr Zwiernik.

October 17, 2017: **Jose Israel Rodriguez** University of Chicago

Title: Trace tests in numerical algebraic geometry

Abstract: Numerical algebraic geometry uses numerical algorithms to study algebraic varieties, which are sets defined by polynomial equations.
It is becoming a core tool in applications of algebraic geometry outside of mathematics. Its fundamental concept is a witness set which gives a representation of a variety that may be manipulated on a computer. The trace test is used to verify that a witness set is complete and provide stopping criteria for algorithms. In this talk we discuss witness sets and how the trace test is used in applications. If time permits we show how a dimension reduction leads to a practical trace test for algebraic varieties in product spaces, which is the natural setting for solutions of parameterized polynomial systems. A background in algebraic geometry will not be assumed.

October 31, 2017: **Elisa Perrone** MIT

Title: The geometry of discrete copulas

Abstract: Discrete copulas are fascinating geometric objects of great importance for empirical modeling in the applied sciences. In this work we analyze mathematical features of discrete copulas, i.e. restrictions of copula functions to non-square uniform grid domains. In particular, we first introduce discrete copulas and briefly discuss their applications to dependence modeling. Then, we highlight fundamental connections between copulas and discrete geometry. Finally, we present a geometric approach to describe families of discrete copulas through the properties of their associated polytopes. This talk is based on a joint work with L.Solus (KTH, Sweden) and C. Uhler (MIT, USA).

November 14, 2017: **Anirbit Mukherjee** Johns Hopkins University

Title: Mathematics of Neural Networks

Abstract: A plethora of exciting mathematics questions have gotten raised in trying to explain the resurgence of neural networks in being able to execute complex artificial intelligence tasks. In this talk I will give a brief overview of some of the questions that me and Amitabh Basu (with other collaborators) have been exploring. We will start from the basic definition of neural networks and will describe various results that we have gotten about the space of functions that these "architectures" represent. We will particularly focus on (1) our recent results (https://arxiv.org/abs/1711.03073) proving a first of its kind lower bounds on the size of high depth neural circuits representing certain Boolean functions and (2) our results trying to formalized the connection between autoencoders and sparse coding (https://arxiv.org/abs/1708.03735).

November 21, 2017: **Daniel Bernstein** North Carolina State University

Title: Algebraic geometry of low-rank matrix completion

Abstract: Given a subset of entries of a matrix, the low-rank matrix completion problem asks for the values of the missing entries that minimize rank. This problem and related ones appear in many applications including collaborative filtering (e.g. the ''Netflix problem''), computer vision, and certifying existence of MLEs in Gaussian graphical models. In this talk, I will discuss the algebraic-geometric approach to this problem, including some classification results about relevant algebraic matroids, and complications that arise when one requires that the completion be real-valued.

November 28, 2017: **Heather Harrington** Oxford

Title: Computational algebraic geometry for topological data analysis

Abstract: I will present how computational algebra can be used to study problems
arising in topological data analysis. I will motivate each of these
problems using applications. Specifically I will discuss two ongoing
projects, one using numerical algebraic geometry to sample points on
real algebraic varieties. The other project proposes new interpretable
invariants for multi-parameter persistent homology using computational
commutative algebra.

December 5, 2017: **David Spivak** MIT

Title: Diagrams for tensor networks and systems of nonlinear equations

Abstract: The same category-theoretic formalism describes both tensor networks and systems of nonlinear relations (including nonlinear equations, inequalities, etc.). More precisely, there is an operad, called Cospan, for which both tensor networks and nonlinear systems are algebras. A morphism between these algebras provides a method for approximating solutions to nonlinear systems using tensor arithmetic. I will explain these ideas, not assuming any background in category theory.

December 12, 2017: **Yang Qi** University of Chicago

Title: Geometry of tensor approximations and tensor decompositions

Abstract: Tensors are closely related to secant varieties. It is known the affine cone of the r-th secant variety of the Segre variety is the set of tensors whose border rank is less than or equal to r. Similarly, we have a geometric interpretation of symmetric tensors. By studying the geometry of these secant varieties, we can derive interesting properties of tensors. In this talk, we will show a general complex (symmetric) tensor has a best (symmetric) rank-r approximation, and we will study the relations between the (border) rank and the symmetric (border) rank of a general symmetric tensor. The talk is based on joint works with Lek-Heng and Mateusz Michalek.