Fall 2022
August 30, 2022 : [in-person]
Speaker:
Emily McMillon, Rice University
Title: Algebraic Connections between Cosets, Syndromes, and Absorbing Sets
Abstract: Mathematical coding theory addresses the problem of transmitting information reliably
and efficiently across noisy channels. Low-density parity-check (LDPC) codes are graph-based codes whose
representation, encoding, and/or decoding can be visualized using a sparse graph representation, called
a Tanner graph. Absorbing sets are combinatorial structures in a code’s Tanner graph that have been shown
to characterize iterative decoder failure of LDPC codes. Previously, absorbing sets have been regarded
as combinatorial, graph-theoretic objects, but in this talk, we demonstrate a novel algebraic
connection between absorbing sets and the cosets and syndromes of their support vectors.
September 6, 2022 : [in-person]
Speaker:
Kenneth Chiu, Rice University
Title: Arithmetic sparsity in mixed Hodge settings
Abstract: We obtain a subpolynomial count of integral Laurent polynomials with fixed data.
We also obtain a subpolynomial count in a more general setting involving mixed period mapping.
It uses simplicial methods in singularity theory and is based on the recent theory developed by
Brunebarbe-Maculan and Ellenberg-Lawrence-Venkatesh (in the pure Hodge setting) about constructing
a certain cover and counting points in it.
Special day, time, location:
Wednesday September 21, 2022 at 3pm, W212 George R. Brown Hall : [in-person]
Speaker:
Quan Chen, Ohio State University
Title: Unitary braided tensor categories from operator algebras
Abstract: Given a W^*-category C, we construct a unitary braided tensor category End_loc(C) of local
endofunctors on C, which is a new construction of a braided tensor category associated with an arbitrary W^*-category.
For the W*-category of finitely generated projective modules over a von Neumann algebra M, this yields a unitary
braiding on Popa's χ~(M), which extends Connes' χ(M) and Jones' kappa invariant.
Given a finite depth inclusion M_0\subset M_1 of non-Gamma II_1 factors, we show that χ~(M_\infty) is equivalent to
the Drinfeld center of the standard invariant, where M_infty is the inductive limit of the Jones tower of basic
construction. This is joint work with Corey Jones and David Penneys (arXiv: 2111.06378)
October 4, 2022 : [in-person]
Speaker:
Kalyani Kansal, Johns Hopkins University
Title: Intersections of irreducible components of the Emerton-Gee stack for GL_2
Abstract: Let K be a finite extension of Q_p. In a 2020 paper, Caraiani, Emerton, Gee and Savitt constructed
a moduli stack of two dimensional mod p representations of the Galois group of K. We compute criteria for codimension
one intersections of the irreducible components of this stack, and interpret them in sheaf-theoretic terms. We also
give a cohomological criterion for the number of top-dimensional components in a codimension one intersection.
October 18, 2022 : [online]
Speaker:
Austen James, Rice University
Title: An algorithm for computing Brauer groups of cubic surfaces via
reduction mod p
Abstract: For arithmetic geometers, the Brauer group of a variety can give us
insight into the existence of rational points. In fact, it is
conjectured that a Brauer-Manin obstruction is the only possible
obstruction to the Hasse principle for del Pezzo surfaces.
Unfortunately for us, the Brauer group is rather sneaky and direct
computation can be extremely difficult and time consuming. In this
talk, we present an algorithm for computing the Brauer group of cubic
surfaces indirectly, via reduction mod p at many primes. Along the
way, we will dip our toes into the world of Bayesian statistics and
debate the merits of certainty versus efficiency!
October 25, 2022 : [in-person]
Speaker:
David Helm, Imperial College London
Title: Local Langlands correspondences in families
Abstract: The local Langlands correspondence predicts a finite-to-one map between isomorphism classes of
irreducible representations of a p-adic reductive group G on one hand, and so-called Langlands parameters for G on
the other. Langlands parameters occur naturally in families, and given such a family there has been considerable
recent interest in constructing corresponding families of representations of G that "interpolate the local Langlands
correspondence" over the family. Such constructions appear in Emerton's description of the completed cohomology of
the modular tower, and more recently in various formulations of a "categorical local Langlands correspondence".
We explain one approach towards the construction of such families; if time permits we will also explain potential
applications in the categorical local Langlands correspondence.
November 1, 2022 : [in-person]
Speaker:
Juanita Duque-Rosero , Dartmouth College
Title: Quadratic Chabauty: geometric and explicit
Abstract: Geometric quadratic Chabauty is a method, pioneered by Edixhoven and Lido ['21], whose goal is to
determine the rational points on a nice curve X. The main tools that this method uses are p-adic analysis and
Gm-torsors over the Jacobian of X. In this talk, I will give an overview of the method, focusing on explicit
computations. I will also present a comparison theorem to the (original) method for quadratic Chabauty.
Finally, we will look at a specific example of a modular curve where the method of geometric quadratic Chabauty
can be used. This is joint work with Sachi Hashimoto and Pim Spelier.
November 8, 2022 : [in-person]
Speaker:
Anthony Gomez Fonseca, University of Notre Dame
Title: Counting cycles in the Tanner graph of QC-LDPC codes
Abstract: In the literature, the relationship between the short cycle distribution and the code’s performance
has been established and used to design good codes. Unfortunately, counting the number of short cycles in a general
graph is known to be a difficult problem. Some algorithms have been devised for counting short cycles in Tanner graphs,
but their complexities depend on the number of edges, the number of vertices, and even the number of cycles, which may
itself increase exponentially with the number of vertices. In this talk, we present a strategy to count the number
of short cycles in the Tanner graph of quasi-cyclic (QC) low-density parity-check (LDPC) codes using the polynomial
parity-check matrix H. Our approach seems to reduce significantly the complexity of the computations when compared
to other approaches. This is exemplified in the discussion.
November 15, 2022 : [in-person]
Speaker:
Xingting Wang, Howard University
Title: Quantum symmetric groups and their twistings
Abstract: We discuss quantum symmetries of noncommutative projective spaces. We introduce
the notion of Manin’s universal quantum groups as the realization of quantum symmetries
and investigate the behaviors of these quantum groups under various deformations of the
underlying noncommutative spaces.
November 29, 2022,
Special time at Noon, 427 HBH : [in-person]
Speaker:
Zachary Spaulding, Rice University
Title: Rationality of Reductions of Cubic Surfaces
Abstract: In the world of birational geometry, rational varieties are the simplest class of varieties.
Questions about rationality were central in classical algebraic geometry and still remain an active area of
research in the modern world. In this talk, we will introduce cubic surfaces, a particularly nicely behaved
class of algebraic surfaces, and their reductions modulo primes. Then, in order to determine when a given
reduction of a cubic surface is rational, we will spend some time discussing the Picard group of cubic
surfaces and its relation to Weyl groups. Finally, we will upgrade these ideas to obtain asymptotic results
on the rationality of the reductions of cubic surfaces. If time permits, we will discuss the generalizations
to other classes of closely related surfaces. Ideas and examples will be emphasized throughout the talk.
December 6, 2022 : [in-person]
Speaker:
Alexander Betts, Harvard University
Title: A Faltings--Mordell Theorem for Selmer sections
Abstract: In 1983, shortly after Faltings’ resolution of the Mordell Conjecture, Grothendieck formulated
his famous Section Conjecture, positing that the set of rational points on a projective curve Y of genus at
least two should be equal to a certain section set defined in terms of the etale fundamental group of Y.
To this day, this conjecture remains wide open, with only a small handful of very special examples known.
In this talk, I will discuss recent work with Jakob Stix, in which we proved a Mordell-like finiteness
theorem for the "Selmer" part of the section set for any smooth projective curve Y of genus at least 2
over the rationals. This generalises the Faltings--Mordell Theorem, and implies strong constraints on
the finite descent locus from obstruction theory. The key new idea in our proof is an adaptation of the
recent proof of Mordell by Lawrence and Venkatesh to the study of the Selmer section set.
Spring 2022
Learning Group Information:
AG Group, HBH 227 3:00pm - 4:00pm, Details [here]
QA Group, HBH 442 3:00pm - 4:00pm, Details [here]
January 19, 2022 : QA Speaker [online]
Speaker:
Marcelo Aguiar, Cornell University
Title: Lie theory relative to a hyperplane arrangement
Abstract: A result due to Joyal, Klyachko, and Stanley relates free Lie algebras
to partition lattices. We will discuss the precise relationship and
interpret the result in terms of the braid hyperplane arrangement.
We will then extend this result to arbitrary (finite, real, and central) hyperplane arrangements,
and do the same with several additional aspects of classical Hopf-Lie theory.
The Tits monoid of an arrangement, and the notion of lune, play central roles in the discussion.
This is part of joint work with Swapneel Mahajan.
January 26, 2022 : Learning Groups - see above
February 2, 2022 : AG Speaker [online]
Speaker:
Sachi Hashimoto, Boston University
Title: Integral points on elliptic curves using p-adic Gross--Zagier
Abstract: Faltings' theorem is a non-explicit theorem that states that there are finitely many rational points on
nice projective curves of genus at least 2 (and also implies there are finitely many integral points on genus 1 affine curves).
The quadratic Chabauty method makes explicit some cases of Faltings' theorem. By studying p-adic heights of points of the Jacobian,
we obtain locally analytic functions that cut out a finite set of p-adic points containing these rational or integral points on the curve.
In this talk, I will explain how we can leverage information from p-adic Gross--Zagier formulas to compute these locally analytic
functions without directly knowing any points on the Jacobian in the simplest case of rank 1 elliptic curves. These Gross--Zagier
formulas relate analytic quantities, special values of p-adic L-functions, to invariants of algebraic cycles, the p-adic height and
logarithm of Heegner points.
February 9, 2022 : Learning Groups - see above
February 16, 2022 : QA Speaker [online]
Speaker:
Ryan Kinser, University of Iowa
Title: Quantum symmetries of algebras related to quivers [
talk notes]
Abstract: The term "quantum symmetries" in this talk refers to Hopf actions of quantum groups, and related Hopf algebras, on
other algebras. While quantum symmetries of field extensions, polynomial rings, and their deformation have been explored for several
decades, only recently have quantum symmetries of algebras related to quivers received close attention. This talk will survey results on
this topic spanning several works by combinations of authors including Berrizbeitia, Etingof, Oswald, Walton, and the speaker.
February 23, 2022 : Learning Groups - see above
March 2, 2022 : AG Speaker [online]
Speaker:
Rosa Winter, King's College London
Title: Density of rational points on del Pezzo surfaces of degree 1
Abstract: Let X be an algebraic variety over an infinite field k. In arithmetic geometry we are interested in the set X(k) of k-rational
points on X. For example, is X(k) empty or not? And if it is not empty, is X(k) dense in X with respect to the Zariski topology?
Del Pezzo surfaces are surfaces classified by their degree d, which is an integer between 1 and 9 (for d ≥ 3, these are the smooth surfaces of
degree d in P^d). For del Pezzo surfaces of degree at least 2 over a field k, we know that the set of k-rational points is Zariski dense
provided that the surface has one k-rational point to start with (that lies outside a specific subset of the surface for degree 2). However,
for del Pezzo surfaces of degree 1 over a field k, even though we know that they always contain at least one k-rational point, we do not know
if the set of k-rational points is Zariski
dense in general.
I will talk about density of rational points on del Pezzo surfaces, state what is known so far, and show a result that is joint work with
Julie Desjardins, in which we give sufficient and necessary conditions for the set of k-rational points on a specific family of del Pezzo
surfaces of degree 1 to be Zariski dense, where k is finitely generated over Q.
March 9, 2022 : Learning Groups - see above
March 16, 2022 : QA Speaker [online]
Francesca Gandini, Kalamazoo College
Title: Constructive Invariant Theory in the exterior algebra (and beyond?)
Abstract: When we consider the action of a finite group on a polynomial ring, an invariant is a polynomial unchanged by the action.
Noether's Degree Bound states that in characteristic zero the maximal degree of a minimal generating invariant is bounded above by the order
of the group. In (commutative) invariant theory, Derksen showed that the generators of the Hilbert ideal can be found via elimination theory
from the vanishing ideal of a subspace arrangement. We show that the same approach works over the exterior algebra and prove Noether's Degree
Bound in this context. Our methods rely on a bound on the Castelnuovo-Mumford regularity of intersections of linear ideals in the exterior algebra,
which we proved in previous work using tools from combinatorial representation theory. We also show a transference of bounds from the symmetric
algebra to the exterior algebra using these tools. A bound on invariant skew polynomials in the exterior algebra also bounds some square-free
invariants in the (-1)-skew algebra and motivates future investigations in the theory of skew polarization.
March 23, 2022 : Learning Groups - see above
March 30, 2022 : AG Speaker [in-person]
Speaker:
Renee Bell, University of Pennsylvania
Title: Monodromy of Tamely Ramified Covers of Curves
Abstract: The étale fundamental group \pi_1^{et} in algebraic geometry
formalizes an analogy between Galois theory and topology, extending
our intuition to spaces in which loops, as defined traditionally, do
not yield meaningful information. For a curve X over an algebraically
closed field of characteristic 0, finite quotients of $\pi_1^{et}$ can
be described solely in topological terms, but in characteristic p,
dramatic differences and new phenomena have inspired many conjectures,
including Abhyankar's conjectures. Let k be an algebraically closed
field of characteristic p and let X be the projective line over k with
three points removed. In joint work with Booher, Chen, and Liu, we
show that for each prime p ≥ 5, there are families of tamely ramified
covers with monodromy the symmetric group S_n or alternating group A_n
for infinitely many n, producing these covers from moduli spaces of
elliptic curves, and relating the fiber of these covers to the Markoff
surface.
April 6, 2022 : Learning Groups - see above
April 13, 2022 : QA Speaker [in-person]
Speaker:
Chelsea Walton, Rice University
Title: Modular categories: What are they, why care, and what have I done with them?
Abstract: In this talk, I will answer the questions in the title; the last question pertains to recent work with Robert Laugwitz (ArXiv: 2010.11872, 2202.08644).
April 20, 2022 :
Colloquium Speaker (still at 3pm) [online]
Stephan Ramon Garcia, Pomona College
Title: Factorization lengths in numerical semigroups
Abstract: Numerical semigroups are simple combinatorial objects that lead to deep and subtle questions. We answer in one fell swoop virtually all asymptotic questions about factorization lengths in numerical semigroups. Surprisingly, this uses tools from complex, harmonic, and functional analysis, probability theory, algebraic combinatorics, and computer-aided design! Our results yield uncannily accurate predictions that agree with numerical computations, along with some totally unexpected byproducts.
Partially supported by NSF Grants DMS-1800123 and DMS-2054002. Joint work with A. Böttcher, M. Omar, C. O'Neill, and undergraduate students G. Udell ('21), T. Wesley ('21), S. Yih ('18).
April 27, 2022 : AG Speaker [online]
Speaker:
Carlos Rivera, University of Washington
Title: Persistence of the Brauer-Manin obstruction for cubic surfaces
Abstract: Let X be a smooth cubic hypersurface over a field k.
Cassels and Swinnerton-Dyer have conjectured that X has a
k-rational point as soon as it has a 0-cycle of degree 1 or,
equivalently, as soon as X has a closed point of degree prime to
3. In 1974, D. Coray showed several results in this direction
including, in the case of cubic surfaces, that the existence of a
closed point of degree prime to 3 implies the existence of a closed
point of degree 1, 4 or 10. In this talk, for k a global
field and X a cubic surface, we will show that a Brauer-Manin
obstruction to the existence of k-points on X will persist over
every extension L/k with degree prime to 3. Therefore proving
that the conjecture of Colliot-Thélène and Sansuc on the
sufficiency of the Brauer-Manin obstruction for cubic surfaces implies
the conjecture of Cassels and Swinnerton-Dyer in this case. This is
joint work with Bianca Viray.
Fall 2021
Learning Group Information:
AG Group, 423 HBH, 3:00pm - 4:00pm, Details [here]
QA Group, 427 HBH, 3:00pm - 4:00pm, Details [here]
August 25, 2021 : Learning Groups [in-person]- see above
September 1, 2021 : AG Speaker [in-person]
Tony Várilly-Alvarado, Rice University
Title: Rational surfaces and locally recoverable codes
Abstract: Motivated by large-scale storage problems around data loss, a budding branch of coding theory has surfaced in the
last decade or so, centered around locally recoverable codes. These codes have the property that individual symbols in a codeword
are functions of other symbols in the same word. If a symbol is lost (as opposed to corrupted), it can be recomputed, and hence a
code word can be repaired. Algebraic geometry has a role to play in the design of codes with locality properties. In this talk I will
explain how to use algebraic surfaces birational to the projective plane to both reinterpret constructions of optimal codes already
found in the literature, and to find new locally recoverable codes, many of which are optimal (in a suitable sense). This is joint
work with Cecília Salgado and Felipe Voloch.
September 8, 2021 : Learning Groups - see above
September 15, 2021 : QA Speaker [in-person]
Sean Sanford, Indiana University
Title: Fusion Categories over Non–Algebraically Closed Fields
Abstract: Much of the early work on Fusion Categories was inspired by physicists desire for rigorous foundations of topological
quantum field theory. One effect of this was that base fields other than the complex numbers were rarely considered, if at all. The
relevant features of $\mathbb C$ that make the theory work are the fact that it is characteristic zero, and algebraically closed.
This talk will focus on the interesting things that can be found when the algebraically closed requirement is removed. The content
will be introductory, with lots of examples.
September 22, 2021 : Learning Groups - see above
September 29, 2021 : AG Speaker [in-person]
Alex Barrios, Carleton College
Title: Local data of elliptic curves with prescribed isogeny graph
Abstract: Given a rational elliptic curve, one can use Tate's algorithm to compute the following local data
at each rational prime: the local conductor exponent, the Néron-Kodaira type, and the local Tamagawa number. This
talk will investigate how the local data changes between two isogenous elliptic curves. We will focus on how the
Néron-Kodaira type and the local Tamagawa number change under isogeny. This has been well-studied in the literature
but remains open for special cases corresponding to a 2- or 3-isogeny. We aim to answer the remaining cases by
explicitly classifying the local data of elliptic curves with a prescribed non-trivial isogeny graph. This is
joint work with Darwin Chimarro, Manami Roy, Nandita Sahajpal, Bella Tobin, and Hanneke Wiersema.
October 6, 2021 : Learning Groups - see above at 3pm
Xingting Wang, Howard University (
Special time at 4pm)
Title:
Can a Zhang twist be a cocycle twist?
Abstract: In this talk, we will discuss two different twists of algebra structures in the literature, namely Zhang twist
and 2-cocycle twist. We will provide sufficient conditions for them to coincide by introducing the notion of a twisting pair.
We will explore twisting pairs for many well-known examples of Hopf algebras including Manin’s universal quantum group from quantum
symmetry and FRT construction from a solution of the quantum Yang-Baxter equation. This is joint work with Hongdi Huang, Van C. Nguyen, Charlotte Ure and Kent B. Vashaw.
October 13, 2021 : QA Speaker [online]
Zachary Dell, Ohio State University
Title: A characterization of braided enriched monoidal categories
Abstract: Monoidal categories enriched in symmetric monoidal categories are well studied in the literature.
In recent years attention has been given to the case where the enriching category is merely braided. In this talk I will
give an overview of the existing results characterizing such categories in terms of braided oplax monoidal functors into
the Drinfeld centers of ordinary monoidal categories and describe how this construction extends to an equivalence of
2-categories.
October 20, 2021 : Learning Groups - see above
October 27, 2021 : AG Speaker [online]
Elisa Bellah, University of Oregon
Title: Bounding Lifts of Markoff Triples mod p
Abstract: In 2016, Bourgain, Gamburd, and Sarnak proved that Strong Approximation holds for the Markoff surface in
most cases. That is, the modulo p solutions to the equation x^2+y^2+z^2=3xyz are covered by the integer points for most primes p.
In this talk, we will discuss how the algorithm given in the paper of Bourgain, Gamburd, and Sarnak can be used to obtain upper
bounds on lifts of Markoff triples modulo p. We will also discuss ongoing work to improve these bounds on average by assuming
the Markoff mod p graphs form an expander family. This is joint work with Elena Fuchs and Lynnelle Ye.
November 3, 2021 : Learning Groups - see above
November 10, 2021 : QA Speaker [online]
Agustina Czensky, University of Oregon
Title: On low rank odd-dimensional MTCs
Abstract:
Classification of fusion categories by rank is an open (and seemingly out of reach) problem. For modular tensor categories (MTCs),
Bruillard, Ng, Rowell and Wang showed in 2016 that there are finitely many of a fixed rank. Bruillard and Rowell also showed that
odd-dimensional MTCs are pointed up to rank 11, which means that they can be classified by group-theoretical data. In this talk
we will describe joint work with Julia Plavnik were we contribute to this classification problem. We will discuss some useful results
for the classification of low-rank odd-dimensional MTCs, and expand on their classification for ranks 15 to 23.
November 17, 2021 : Learning Groups - see above
November 24, 2021 : (no activity)
December 1, 2021 : AG Speaker [online]
Humberto Diaz, Washington University
Title: The Hodge Conjecture for varieties with small Chow group
Abstract: The integral Hodge conjecture asserts that every integral Hodge class is the class of an algebraic cycle. It has been known
to be false since the counterexamples of Atiyah and Hirzebruch. Since then, many other types of counterexamples have been produced. In this talk,
I will report on recent work that gives yet another type of counterexample, this time arising from a variety whose Chow group of algebraic cycles
is as small as possible.
December 8, 2021 : QA Speaker [online]
Florencia Orosz, University of Denver
Title: Tensor categories arising from the Virasoro algebra
Abstract: In this talk we will discuss the tensor structure associated with certain representations of the Virasoro algebra. In particular,
we will show that there is a braided tensor category structure on the category of C1-cofinite modules for the Virasoro vertex operator algebras of
arbitrary central charge. This talk is based on joint work with Thomas Creutzig, Cuibo Jiang, David Ridout and Jinwei Yang.
