Seminars and Colloquia by Series

Fun with Mac Lane valuations

Series
Algebra Seminar
Time
Monday, February 11, 2019 - 13:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Andrew Obus – Baruch College, CUNY –
Mac Lane's technique of "inductive valuations" is over 80 years old, but has only recently been used to attack problems about arithmetic surfaces. We will give an explicit, hands-on introduction to the theory, requiring little background beyond the definition of a non-archimedean valuation. We will then outline how this theory is helpful for resolving "weak wild" quotient singularities of arithmetic surfaces, as well as for proving conductor-discriminant inequalities for higher genus curves. The first project is joint work with Stefan Wewers, and the second is joint work with Padmavathi Srinivasan.

Kazhdan-Lusztig theory for matroids

Series
Algebra Seminar
Time
Monday, February 4, 2019 - 12:50 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Botong Wang – University of Wisconsin-Madison –
Matroids are basic combinatorial objects arising from graphs and vector configurations. Given a vector configuration, I will introduce a “matroid Schubert variety” which shares various similarities with classical Schubert varieties. I will discuss how the Hodge theory of such matroid Schubert varieties can be used to prove a purely combinatorial conjecture, the “top-heavy” conjecture of Dowling-Wilson. I will also report an on-going project joint with Tom Braden, June Huh, Jacob Matherne, Nick Proudfoot on the cohomology theory of non-realizable matroids.

Non-Archimedean Hyperbolicity and Applications

Series
Algebra Seminar
Time
Monday, January 28, 2019 - 12:50 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Jackson Morrow – Emory university –
The conjectures of Green—Griffths—Lang predict the precise interplay between different notions of hyperbolicity: Brody hyperbolic, arithmetically hyperbolic, Kobayashi hyperbolic, algebraically hyperbolic, groupless, and more. In his thesis (1993), W.~Cherry defined a notion of non-Archimedean hyperbolicity; however, his definition does not seem to be the "correct" version, as it does not mirror complex hyperbolicity. In recent work, A.~Javanpeykar and A.~Vezzani introduced a new non-Archimedean notion of hyperbolicity, which ameliorates this issue, and also stated a non-Archimedean variant of the Green—Griffths—Lang conjecture. In this talk, I will discuss complex and non-Archimedean notions of hyperbolicity as well as some recent progress on the non-Archimedean Green—Griffths—Lang conjecture. This is joint work with Ariyan Javanpeykar (Mainz) and Alberto Vezzani (Paris 13).

The dimension of an amoeba

Series
Algebra Seminar
Time
Friday, January 25, 2019 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Chi Ho Yuen – University of Bern –
An amoeba is the image of a subvariety X of an algebraic torus under the logarithmic moment map. Nisse and Sottile conjectured that the (real) dimension of an amoeba is smaller than the expected one, namely, two times the complex dimension of X, precisely when X has certain symmetry with respect to toric actions. We prove their conjecture and derive a formula for the dimension of an amoeba. We also provide a connection with tropical geometry. This is joint work with Jan Draisma and Johannes Rau.

Canonical measures on graphs and a Kazhdan’s theorem

Series
Algebra Seminar
Time
Wednesday, December 5, 2018 - 14:30 for 1 hour (actually 50 minutes)
Location
Skiles 249
Speaker
Farbod Shokrieh – University of Copenhagen –
Classical Kazhdan's theorem for Riemann surfaces describes the limiting behavior of canonical (Arakelov) measures on finite covers in relation to the hyperbolic measure. I will present a generalized version of this theorem for metric graphs. (Joint work with Chenxi Wu.)

Linear dependence among powers of polynomials

Series
Algebra Seminar
Time
Monday, December 3, 2018 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Bruce Reznick – University of Illinois, Urbana Champaign
One variation of the Waring problem is to ask for the shortest non-trivial equations of the form f_1^d + ... + f_r^d = 0, under various conditions on r, d and where f_j is a binary form. In this talk I'll limit myself to quadratic forms, and show all solutions for r=4 and d=3,4,5. I'll also give tools for you to find such equations on your own. The talk will touch on topics from algebra, analysis, number theory, combinatorics and algebraic geometry and name-check such notables as Euler, Sylvester and Ramanujan, but be basically self-contained. To whet your appetite: (x^2 + xy - y^2)^3 + (x^2 - xy - y^2)^3 = 2x^6 - 2y^6.

Low degree points on curves

Series
Algebra Seminar
Time
Friday, November 30, 2018 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Isabel Vogt – Massachusetts Institute of Technology –
In this talk we will discuss an arithmetic analogue of the gonality of a nice curve over a number field: the smallest positive integer e such that the points of residue degree bounded by e are infinite. By work of Faltings, Harris--Silverman and Abramovich--Harris, it is understood when this invariant is 1, 2, or 3; by work of Debarre-Fahlaoui these criteria do not generalize. We will focus on scenarios under which we can guarantee that this invariant is actually equal to the gonality using the auxiliary geometry of a surface containing the curve. This is joint work with Geoffrey Smith.

Cofinality of formal Gubler models

Series
Algebra Seminar
Time
Friday, November 16, 2018 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Tyler Foster – Florida State University
Let K be a non-trivially valued non-Archimedean field, R its valuation subring. A formal Gubler model is a formal R-scheme that comes from a polyhedral decomposition of a tropical variety. In this talk, I will present joint work with Sam Payne in which we show that any formal model of any compact analytic domain V inside a (not necessarily projective) K-variety X can be dominated by a formal Gubler model that extends to a model of X. This result plays a central role in our work on "structure sheaves" on tropicalizations and our work on adic tropicalization. If time permits I will explain some of this work.

Chi-y genera of generic intersections in algebraic tori and refined tropicalizations

Series
Algebra Seminar
Time
Friday, October 26, 2018 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Andreas Gross – Colorado State University
An algorithm to compute chi-y genera of generic complete intersections in algebraic tori has already been known since the work of Danilov and Khovanskii in 1978, yet a closed formula has been given only very recently by Di Rocco, Haase, and Nill. In my talk, I will show how this formula simplifies considerably after an extension of scalars. I will give an algebraic explanation for this phenomenon using the Grothendieck rings of vector bundles on toric varieties. We will then see how the tropical Chern character gives rise to a refined tropicalization, which retains the good properties of the usual, unrefined tropicalization.

Hyperfields, Ordered Blueprints, and Moduli Spaces of Matroids

Series
Algebra Seminar
Time
Friday, October 19, 2018 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Matt Baker – Georgia Tech –
I will begin with a gentle introduction to hyperrings and hyperfields (originally introduced by Krasner for number-theoretic reasons), and then discuss a far-reaching generalization, Oliver Lorscheid’s theory of ordered blueprints. Two key examples of hyperfields are the hyperfield of signs S and the tropical hyperfield T. An ordering on a field K is the same thing as a homomorphism to S, and a (real) valuation on K is the same thing as a homomorphism to T. In particular, the T-points of an ordered blue scheme over K are closely related to Berkovich’s theory of analytic spaces.I will discuss a common generalization, in this language, of Descartes' Rule of Signs (which involves polynomials over S) and the theory of Newton Polygons (which involves polynomials over T). I will then introduce matroids over hyperfields (as well as certain more general kinds of ordered blueprints). Matroids over S are classically called oriented matroids, and matroids over T are also known as valuated matroids. I will explain how the theory of ordered blueprints and ordered blue schemes allow us to construct a "moduli space of matroids”, which is the analogue in the theory of ordered blue schemes of the usual Grassmannian variety in algebraic geometry. This is joint work with Nathan Bowler and Oliver Lorscheid.

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