Mathematics and Statistics

TAG Seminar


In Spring 22 the Topology, Algebra, and Geometry Seminar is held 1:30 - 2:30 PM on Fridays, primarily via Zoom. To receive a link to the Zoom meeting and be notified of future talks, please email To download notes from a given day's lecture, please click on the links below.

Date Speaker Title Abstract / NOTES
Jan 28 & Feb 4 Ivan Soprunov Plücker-type inequalities for mixed areas and intersection numbers of tropical curves Consider $n$ planar tropical curves and count their pairwise intersection numbers. It turns out that starting with $n=4$ these ${n\choose 2}$ numbers satisfy certain quadratic inequalities similar to the well-known Plücker relations for the Grassmannian. In the first part, I will state the result for $n=4$ and explain the relation between this problem and the problem of describing mixed area configuration spaces. In the second part, we will look at some ideas of the proof as well as a graph representation of spaces defined by Plücker-type inequalities.
Nov 12 & 19 Tianran Chen Fireflies, power plants, and convex polytopes The Kuramoto equation has been used extensively in the study of mysterious phenomena of spontaneous synchronization in biology (such as synchronized flashing of fireflies) and chemistry (such as chemical oscillators). The closely related power-flow equation is used to model the synchronization of power generators as well as the balancing of power generation and distribution, and it is studied intensively in electric engineering. In this talk, we explore how discrete geometry, Newton polytopes, and the theory of Bernshtein-Kushnirenko-Khovanskii bound brings new insights into the study of these two equations. This talk includes my joint work with Robert Davis and Evgeniia Korchevskaia.
Oct 29 & Nov 5 Eduardo Camps Powers of Principal Q-Borel Ideals Consider $R:=\mathbb{K}[x_1,\ldots,x_n]$ the polynomial ring over a field of characteristic zero and $Q$ a poset over $\{x_1,\ldots,x_n\}$. Given a monomial $m\in S$, we say that the ideal $I=( x_j*m/x_i\ |\ x_i|m,\ x_j\leq_Q x_i)$ is a principal $Q$-Borel ideal. These are a generalization of the concept of Borel ideals and the purpose of this talk is to get as much information as possible of the ideal using just the information to build it, this is the poset $Q$ and the monomial $m$. Specifically, we are going to look the symbolic powers of this ideals and we are going to prove that these coincide with the regular powers.
Oct 15 & 22 Delio Jaramillo-Velez F-thresholds and test ideals of Thom-Sebastiani type polynomials In this talk I am gonna make some comments about the roots of test ideals and F-thresholds. After this I am gonna focus on the case of the test ideal of a hypersurface. Finally,  I'll explain part of a joint work with Luis Núñez-Betancourt and Manuel González Villa about the case of the F-thresholds of a Thom-Sebastiani polynomial.
Sep 24 & Oct 1 Yuriko Pitones Resolution of a linear code In this talk we will show two methods to associate a free resolution to linear codes. The first, via matroids and the associated Stanley-Reisner ideal, which gives us information about the generalized Hamming weights. The second, using a binomial ideal and Gröbner basis theory.


Date Speaker Title Abstract / NOTES
Apr 2, 9 & 23 Ivan Soprunov On the number of F_q-zeros of families of sparse polynomials Fix a lattice polytope P and consider the family of polynomials whose monomials have exponents in P and whose coefficients are arbitrary elements of a finite field F_q. Our goal is to estimate the maximum number of F_q-zeros of the polynomials in such a family in terms of geometric/combinatorial invariants of P. When q is large, the maximum is given by polynomials with the largest number of factors. On the combinatorial side, this corresponds to “maximal Minkowski decompositions” inside P. I will first explain this connection and review the previously known case of bivariate polynomials (i.e. dim P=2) as well as the “classical” case of n-variate polynomials of given degree. I will then focus on the case of trivariate polynomials (i.e. dim P=3) where the combinatorics is much richer and new phenomena arise. The talk is based on a joint work with Kyle Meyer and Jenya Soprunova. (notes)
Mar 5, 12 & 19 Federico Galetto Finite group actions on free resolutions Minimal free resolutions are invariants of finitely generated graded modules over polynomial rings. Under mild conditions, group actions on modules extend to group actions on minimal free resolutions. After reviewing the basics, I will show how to determine characters of finite groups acting on free resolutions. (notes)
Feb 19 Graham Keiper Regularity and h-Polynomial Degree of Toric Ideals of Graphs I will discuss recent work of Favacchio, Van Tuyl and myself which demonstrates that we may construct toric ideals associated with graphs which have regularity r and h-polynomials of degree d for any 4 ≤ r ≤ d. The ideas involved rely on Groebner bases and combinatorics. I will also discuss how we can use this to recover a result of Hibi, Higashitani, Kimura, and O’Keefe.
Jan. 29, Feb. 5 & 12 Steven Gubkin Sheaves on Sites In preparation for understanding the definition of condensed sets (ala Scholze and Clausen), we will motivate and explore the properties of sheaves on a site.  A sheaf of sets on a topological space is a contravariant functor from its poset of open sets to Sets which satisfies a gluing condition.  The category of sheaves on a space is a kind of category called a topos which has extremely nice categorical properties.  Generalizing, we then define a "site" which is a category with a notion of "covers" allowing sheaves to be defined on them.  Sites generalize the poset of opens of a topological space, and sheaves on a site also form a topos. (notes)
Oct. 30 & Nov. 20 Jonathan Scott Commutative Differential Graded Algebras with Lattices Let X be a topological space;  the image of its integral cohomology in its rational cohomology is called its lattice. We define an invariant, the “commutative differential graded algebra with lattice” of X, which encodes its rational homotopy type as well as its lattice structure. This is joint work with Don Stanley and Xiyuan Wang. (notes)
Oct. 9 & 16 Hiram Lopez An introduction to evaluation codes We will see how evaluation codes are related to commutative algebra. (notes)

Sep. 11, 18, & 25

Federico Galetto

Jets of graphs

Inspired by a construction in algebraic geometry, I will define jets of graphs and discuss a few different properties in the context of combinatorial commutative algebra. (notes)

Spring 2020

Date Speaker Title Abstract / NOTES
Jan 31, Feb 7, and Feb 14 Ivan Soprunov Relations between Mixed Volumes Recently Esterov and Gusev showed that the problem of classifying generic sparse polynomial systems which are solvable in radicals reduces to the problem of classifying collections of lattice polytopes of mixed volume up to 4. Given the value of mixed volume m and dimension n, there exist only finitely many collections (P_1,…,P_n) of n-dimensional lattice polytopes of mixed volume m, up to unimodular transformations. One reason for this is that the volume of the Minkowski sum P_1+⋯+P_n is bounded above by O(m^{2^n}), as follows by a direct application of the Aleksandrov-Fenchel inequality. I will show how one can employ more relations between mixed volumes to improve the bound to O(m^n), which is asymptotically sharp. We will also see the exact sharp bound in dimensions 2 and 3. This is joint work with Gennadiy Averkov and Christopher Borger.

Fall 2019

Date Speaker Title Abstract



Greg Lupton (CSU) A Fundamental Group in Digital Topology

Notes from the seminar

Sept. 13th Greg Lupton (CSU) A Fundamental Group in Digital Topology

A \emph{digital image} is a finite subset of the integer lattice in $\mathbb{R}^n$ together with an adjacency relation.  For instance, a 2-dimensional digital image is an abstraction of an actual digital image consisting of pixels.  Then \emph{digital topology} refers to the many and ongoing efforts to use notions from (algebraic) topology to  study and analyze digital images.


  As part of a broad program to bring notions of homotopy theory into the setting of digital topology, we have defined a fundamental group for an arbitrary digital image.  Our construction of this digital fundamental group is intrinsic, in that it is defined in an essentially combinatorial way directly in terms of the digital image; it also differs in key aspects from previously defined versions of the fundamental group in digital topology.


We show that our fundamental group of a digital image is isomorphic to the edge group of the clique complex of the digital image considered as a graph.      The clique complex is a simplicial complex.  As such, its edge group is  isomorphic to the ordinary (topological) fundamental group of its geometric realization.  These identifications mean that many familiar facts about the ordinary topological fundamental group may be translated into their counterparts for our digital fundamental group.   We conclude that the digital fundamental group of any digital circle is $\Z$. We deduce the Seifert-Van Kampen Theorem in the setting of digital topology.  We show that the (digital) fundamental group of every 2-dimensional digital image is a free group.   These results have the potential to be useful in topology-related image processing operations such as thinning and feature extraction. 
Sept. 20th Jonathan Scott (CSU) Algebraic Factorization Systems for Algebras

Model categories were created by Daniel Quillen to provide a general environment in which to do homotopy theory.  The factorizations asked for by the axioms (for example, factoring a morphism into a weak cofibration followed by a fibration) are nowadays assumed to be functorial, or natural.  Emily Riehl showed that, if the fibration half of the factorization had a certain algebraic structure (and the cofibrations a coalgebraic structure), then the solutions to the lifting problems demanded by the model structure can also be made functorial.  Furthermore, she showed that any cofibrantly generated model category (such as the category of chain algebras) has such an algebraic system.

We provide an explicit construction of such an algebraic system for chain algebras, using relatively classical techniques.  

This is joint work with P.-E. Parent (U. Ottawa) and K. Hess (EPFL).

Sept. 27th

Jonathan Scott (CSU) 

Algebraic Factorization Systems for Algebras Cont'd
Oct. 4th Federico Galetto Betti numbers in commutative algebra Lecture Notes
Oct. 18th

Federico Galetto

Linear Quotients PDF icon Lecture Notes
Nov. 15th

Federico Galetto

Star Configurations PDF icon Lecture Notes


Spring 2019

Date Speaker Title Abstract



Viji Thomas (CSU)



We will introduce Kaplansky's conjecture and talk about what all has been proved so far.


Fall 2018

Date Speaker Title Abstract
October 12 Greg Lupton (CSU) Digital Homotopy Theory

I will describe work in progress, joint with John Oprea and Nick Scoville.  An n-dimensional digital image is a finite subset of the integer lattice in R^n, together with an adjacency relation.  For instance, a 2-dimensional digital image is an abstraction of an actual digital image consisting of pixels.  Our work consists of developing notions and techniques from homotopy theory in the setting of digital images.  

In an extensive literature,  a number of authors have introduced concepts from topology into the study of digital images.   But some of these notions, as they appear in the literature, do not seem satisfactory from a homotopy point of view.  Indeed, some of the constructs most useful in homotopy theory, such as cofibrations and path spaces, are absent from the literature.  Working in the digital setting, we develop some basic ideas of homotopy theory, including cofibrations and path fibrations, in a way that seems more suited to homotopy theory.  We illustrate how our approach may be used, for example, to study Lusternik-Schnirelmann category and topological complexity in a digital setting.  One future goal is to develop a characterization of a "homotopy circle" (in the digital setting) using the notion of topological complexity.  This is with a view towards recognizing circles, and perhaps other features, using these ideas.  The talk(s) will include a survey of the basics on topological notions in the setting of digital images. 

October 19 Greg Lupton (CSU) Digital Homotopy Theory Continued
October 26 Greg Lupton (CSU) Digital Homotopy Theory Continued
November 9 Viji Thomas (CSU) A Group theoretical construction We will introduce a group theoretical construction and show its relation to the Schur Multiplier (Second Homology group with coefficients in the integers), second stable homotopy group of the Eilenberg Maclane space and the Bogomolov multiplier. If time permits we will show its relation to a conjecture of Schur and also its relation to Noethers rationality problem.