## Seminars and Colloquia by Series

### Effective Chabauty for symmetric powers of curves

Series
Algebra Seminar
Time
Wednesday, November 19, 2014 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Jennifer ParkMcGill University
Faltings' theorem states that curves of genus g> 1 have finitely many rational points. Using the ideas of Faltings, Mumford, Parshin and Raynaud, one obtains an upper bound on the number of rational points, but this bound is too large to be used in any reasonable sense. In 1985, Coleman showed that Chabauty's method, which works when the Mordell-Weil rank of the Jacobian of the curve is smaller than g, can be used to give a good effective bound on the number of rational points of curves of genus g > 1. We draw ideas from nonarchimedean geometry and tropical geometry to show that we can also give an effective bound on the number of rational points outside of the special set of the d-th symmetric power of X, where X is a curve of genus g > d, when the Mordell-Weil rank of the Jacobian of the curve is at most g-d.

### Moduli of Tropical Plane Curves

Series
Algebra Seminar
Time
Friday, November 14, 2014 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 202
Speaker
Ralph MorrisonBerkeley
Smooth curves in the tropical plane correspond to unimodulartriangulations of lattice polygons. The skeleton of such a curve is ametric graph whose genus is the number of lattice points in the interior ofthe polygon. In this talk we report on work concerning the followingrealizability problem: Characterize all metric graphs that admit a planarrepresentation as a smooth tropical curve. For instance, about 29.5 percentof metric graphs of genus 3 have this property. (Joint work with SarahBrodsky, Michael Joswig, and Bernd Sturmfels.)

### Joint Athens-Atlanta Number Theory

Series
Algebra Seminar
Time
Tuesday, November 4, 2014 - 16:00 for 4 hours (half day)
Location
Skiles 005
Speaker
Arul Shankar and Wei ZhangHarvard University and Columbia University
The Joint Athens-Atlanta Number Theory Seminar meets once a semester, usually on a Tuesday, with two talks back to back, at 4:00 and at 5:15. Participants then go to dinner together.

### Exceptional isogenies between elliptic curves

Series
Algebra Seminar
Time
Monday, November 3, 2014 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
François CharlesMIT and Paris-Sud
We will discuss a proof of the following result: if E and E' are two elliptic curves over a number field, there exist infinitely many places p of k such that the reduction of E and E' modulo p are isogenous. We will explain the relationship with the dynamics of Hecke correspondences on modular curves and the heuristics behind such results.

### F-singularities and weak ordinarity

Series
Algebra Seminar
Time
Monday, October 20, 2014 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Karl SchwedeUniversity of Utah
I will discuss recent work of Bhargav Bhatt, myself and Shunsuke Takagi relating several open problems and generalizing work of Mustata and Srinivas. First: whether a smooth complex variety is ordinary after reduction to characteristic $p > 0$ for infinitely many $p$. Second: that multiplier ideals reduce to test ideals for infinitely many $p$ (regardless of coefficients). Finally, whether complex varieties with Du Bois singularities have $F$-injective singularities after reduction to infinitely many $p > 0$.

### Economics for tropical geometer

Series
Algebra Seminar
Time
Tuesday, October 7, 2014 - 03:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Ngoc Mai TranUT Austin
This talk surveys the connection between economics and tropical geometry, as developed in the paper of Baldwin and Klemperer (Tropical Geometry to Analyse Demand). I will focus on translating concepts, theorems and questions in economics to tropical geometry terms.

### Enumerating Polytropes

Series
Algebra Seminar
Time
Monday, October 6, 2014 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Ngoc Mai TranUT Austin
Polytropes are both ordinary and tropical polytopes. Tropical types of polytropes in \R^n are in bijection with certain cones of a specific Gr\"obner fan in \R^{n^2-n}. Unfortunately, even for n = 5 the entire fan is too large to be computed by existing software. We show that the polytrope cones can be decomposed as the cones from the refinement of two fans, intersecting with a specific cone. This allows us to enumerate types of full-dimensional polytropes for $n = 4$, and maximal polytropes for $n = 5$ and $n = 6$. In this talk, I will prove the above result and describe the key difficulty in higher dimensions.

### Gambling on Massey zero in a dramatic spin of absolute Galois groups

Series
Algebra Seminar
Time
Friday, October 3, 2014 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Ján MinacUniversity of Western Ontario
Similar to the glamour of Las Vegas, the excitement and drama of winning in casinos and falling under the spell of such legends as Frank Sinatra and Dean Martin; is the search for revealing the mystery of absolute Galois groups and their special properties among other profinite groups. The recent, spectacular proof of the Bloch-Kato conjecture by Rost and Voevodsky, with Weibel's patch, and some current and interesting developments involving Massey products, hold great promise and new challenges on the road to understanding the structure of absolute Galois groups. This talk will provide an overview of the subject, and then explain some recent results obtained with Nguyen Duy Tan.

### Goodness-of-fit testing in the Ising Model

Series
Algebra Seminar
Time
Monday, September 29, 2014 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Abraham Martin del CampoIST Austria
Markov bases have been developed in algebraic statistics for exact goodness-of-fit testing. They connect all elements in a fiber (given by the sufficient statistics) and allow building a Markov chain to approximate the distribution of a test statistic by its posterior distribution. However, finding a Markov basis is often computationally intractable. In addition, the number of Markov steps required for converging to the stationary distribution depends on the connectivity of the sampling space.In this joint work with Caroline Uhler and Sarah Cepeda, we compare different test statistics and study the combinatorial structure of the finite lattice Ising model. We propose a new method for exact goodness-of-fit testing. Our technique avoids computing a Markov basis but builds a Markov chain consisting only of simple moves (i.e. swaps of two interior sites). These simple moves might not be sufficient to create a connected Markov chain. We prove that when a bounded change in the sufficient statistics is allowed, the resulting Markov chain is connected. The proposed algorithm not only overcomes the computational burden of finding a Markov basis, but it might also lead to a better connectivity of the sampling space and hence a faster convergence.

### Tropical K_4 curves

Series
Algebra Seminar
Time
Wednesday, September 24, 2014 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Melody ChanHarvard University
This is joint work with Pakwut Jiradilok. Let X be a smooth, proper curve of genus 3 over a complete and algebraically closed nonarchimedean field. We say X is a K_4-curve if the nonarchimedean skeleton G of X is a metric K_4, i.e. a complete graph on 4 vertices.We prove that X is a K_4-curve if and only if X has an embedding in p^2 whose tropicalization has a strong deformation retract to a metric K_4. We then use such an embedding to show that the 28 odd theta characteristics of X are sent to the seven odd theta characteristics of g in seven groups of four. We give an example of the 28 bitangents of a honeycomb plane quartic, computed over the field C{{t}}, which shows that in general the 4 bitangents in a given group need not have the same tropicalizations.