## Seminars and Colloquia by Series

### Tropical Dolbeault cohomology of non-archimedean analytic spaces

Series
Algebra Seminar
Time
Monday, November 20, 2017 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Philipp JellGeorgia Tech
Real-valued smooth differential forms on Berkovich analytic spaces were introduced by Chambert-Loir and Ducros. They show many fundamental properties analogous to smooth real differential forms on complex manifolds, which are used for example in Arakelov geometry. In particular, these forms define a real valued bigraded cohomology theory for Berkovich analytic space, called tropical Dolbeault cohomology. I will explain the definition and properties of these forms and their link to tropical geometry. I will then talk about results regarding the tropical Dolbeault cohomology of varietes and in particular curves. In particular, I will look at finite dimensionality and Poincar\'e duality.

### Local-to-Global lifting to curves in characterstic p

Series
Algebra Seminar
Time
Monday, November 13, 2017 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Renee BellMassachusetts Institute of Technology
Given a Galois cover of curves X to Y with Galois group G which is totally ramified at a point x and unramified elsewhere, restriction to the punctured formal neighborhood of x induces a Galois extension of Laurent series rings k((u))/k((t)). If we fix a base curve Y , we can ask when a Galois extension of Laurent series rings comes from a global cover of Y in this way. Harbater proved that over a separably closed field, this local-to-global principle holds for any base curve if G is a p-group, and gave a condition for the uniqueness of such an extension. Using a generalization of Artin-Schreier theory to non-abelian p-groups, we characterize the curves Y for which this lifting property holds and when it is unique, but over a more general ground field.

### Interpolation problems for curves in projective space

Series
Algebra Seminar
Time
Monday, November 6, 2017 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Isabel VogtMassachusetts Institute of Technology
In this talk we will discuss the following question: When does there exist a curve of degree d and genus g passing through n general points in P^r? We will focus primarily on what is known in the case of space curves (r=3).

### Three-isogeny Selmer groups and ranks of abelian varieties in quadratic twist families

Series
Algebra Seminar
Time
Monday, October 23, 2017 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Robert Lemke OliverTufts University
We determine the average size of the $\phi$-Selmer group in any quadratic twist family of abelian varieties having an isogeny $\phi$ of degree 3 over any number field. This has several applications towards the rank statistics in such families of quadratic twists. For example, it yields the first known quadratic twist families of absolutely simple abelian varieties over $\mathbb{Q}$, of dimension greater than one, for which the average rank is bounded; in fact, we obtain such twist families in arbitrarily large dimension. In the case that $E/F$ is an elliptic curve admitting a 3-isogeny, we prove that the average rank of its quadratic twists is bounded; if $F$ is totally real, we moreover show that a positive proportion of these twists have rank 0 and a positive proportion have $3$-Selmer rank 1. We also obtain consequences for Tate-Shafarevich groups of quadratic twists of a given elliptic curve. This is joint work with Manjul Bhargava, Zev Klagsbrun, and Ari Shnidman.

### Jensen-Pólya Criterion for the Riemann Hypothesis and Related Problems

Series
Algebra Seminar
Time
Monday, October 16, 2017 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Larry RolenGeorgia Tech
In this talk, I will summarize forthcoming work with Griffin, Ono, and Zagier. In 1927 Pólya proved that the Riemann Hypothesis is equivalent to the hyperbolicity of Jensen polynomials for Riemann's Xi-function. This hyperbolicity has been proved for degrees $d\leq 3$. We obtain an arbitrary precision asymptotic formula for the derivatives $\Xi^{(2n)}(0)$, which allows us to prove thehyperbolicity of 100% of the Jensen polynomials of each degree. We obtain a general theorem which models such polynomials by Hermite polynomials. This general condition also confirms a conjecture of Chen, Jia, and Wang.

### Infinite Loop Spaces in Algebraic Geometry

Series
Algebra Seminar
Time
Monday, October 2, 2017 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Elden ElmantoNorthwestern
A classical theorem in modern homotopy theory states that functors from finite pointed sets to spaces satisfying certain conditions model infinite loop spaces (Segal 1974). This theorem offers a recognition principle for infinite loop spaces. An analogous theorem for Morel-Voevodsky's motivic homotopy theory has been sought for since its inception. In joint work with Marc Hoyois, Adeel Khan, Vladimir Sosnilo and Maria Yakerson, we provide such a theorem. The category of finite pointed sets is replaced by a category where the objects are smooth schemes and the maps are spans whose "left legs" are finite syntomic maps equipped with a K​-theoretic trivialization of its contangent complex. I will explain what this means, how it is not so different from finite pointed sets and why it was a natural guess. In particular, I will explain some of the requisite algebraic geometry.Time permitting, I will also provide 1) an explicit model for the motivic sphere spectrum as a torsor over a Hilbert scheme and,2) a model for all motivic Eilenberg-Maclane spaces as simplicial ind-smooth schemes.

### p-adic metric line bundles and integral points on curves

Series
Algebra Seminar
Time
Monday, September 25, 2017 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Amnon BesserGeorgia Tech

Please Note: postponed from September 18

In this talk I first wish to review my work with Balakrishnan and Muller, giving an algorithm for finding integral points on curves under certain (strong) assumptions. The main ingredients are the theory of p-adic height pairings and the theory of p-adic metrized line bundles. I will then explain a new proof of the main result using a p-adic version of Zhang's adelic metrics, and a third proof which only uses the metric at one prime p. At the same time I will attempt to explain why I think this last proof is interesting, being an indication that there may be new p-adic methods for finding integral points.

### Abelian varieties isogenous to Jacobians

Series
Algebra Seminar
Time
Friday, April 28, 2017 - 11:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Ananth ShankarHarvard University
Chai and Oort have asked the following question: For any algebraically closed field $k$, and for $g \geq 4$, does there exist an abelian variety over $k$ of dimension $g$ not isogenous to a Jacobian? The answer in characteristic 0 is now known to be yes. We present a heuristic which suggests that for certain $g \geq 4$, the answer in characteristic $p$ is no. We will also construct a proper subvariety of $X(1)^n$ which intersects every isogeny class, thereby answering a related question, also asked by Chai and Oort. This is joint work with Jacob Tsimerman.

### Curves, Graphs, and Tangent Lines

Series
Algebra Seminar
Time
Monday, April 24, 2017 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Yoav LenUniversity of Waterloo
I will discuss the interplay between tangent lines of algebraic and tropical curves. By tropicalizing all the tangent lines of a plane curve, we obtain the tropical dual curve, and a recipe for computing the Newton polygon of the dual projective curve. In the case of canonical curves, tangent lines are closely related with various phenomena in algebraic geometry such as double covers, theta characteristics and Prym varieties. When degenerating them in families, we discover analogous constructions in tropical geometry, and links between quadratic forms, covers of graphs and tropical bitangents.

### Graph Structure in Polynomial Ideals: Chordal Networks

Series
Algebra Seminar
Time
Friday, April 14, 2017 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Diego CifuentesMIT
We introduce a novel representation of structured polynomial ideals, which we refer to as chordal networks. The sparsity structure of a polynomial system is often described by a graph that captures the interactions among the variables. Chordal networks provide a computationally convenient decomposition of a polynomial ideal into simpler (triangular) polynomial sets, while preserving its underlying graphical structure. We show that many interesting families of polynomial ideals admit compact chordal network representations (of size linear in the number of variables), even though the number of components could be exponentially large. Chordal networks can be computed for arbitrary polynomial systems, and they can be effectively used to obtain several properties of the variety, such as its dimension, cardinality, equidimensional components, and radical ideal membership. We apply our methods to examples from algebraic statistics and vector addition systems; for these instances, algorithms based on chordal networks outperform existing techniques by orders of magnitude.