Seminars and Colloquia by Series

Algebraic methods for maximum likelihood estimation

Series
Algebra Seminar
Time
Monday, April 9, 2018 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 005 or 006
Speaker
Kaie KubjasMIT / Aalto University
Given data and a statistical model, the maximum likelihood estimate is the point of the statistical model that maximizes the probability of observing the data. In this talk, I will address three different approaches to maximum likelihood estimation using algebraic methods. These three approaches use boundary stratification of the statistical model, numerical algebraic geometry and the EM fixed point ideal. This talk is based on joint work with Allman, Cervantes, Evans, Hoşten, Kosta, Lemke, Rhodes, Robeva, Sturmfels, and Zwiernik.

Higher nerves of simplicial complexes

Series
Algebra Seminar
Time
Friday, April 6, 2018 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Hai Long DaoUniversity of Kansas
The nerve complex of an open covering is a well-studied notion. Motivated by the so-called Lyubeznik complex in local algebra, and other sources, a notion of higher nerves of a collection of subspaces can be defined. The definition becomes particularly transparent over a simplicial complex. These higher nerves can be used to compute depth, and the h-vector of the original complex, among other things. If time permits, I will discuss new questions arises from these notions in commutative algebra, in particular a recent example of Varbaro on connectivity of hyperplane sections of a variety. This is joint work with J. Doolittle, K. Duna, B. Goeckner, B. Holmes and J. Lyle.

On wild covers of Berkovich curves and the lifting problem

Series
Algebra Seminar
Time
Monday, March 12, 2018 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Michael TemkinHebrew University
The structure of non-archimedean curves X and their tame covers f:Y-->X is well understoodand can be adequately described in terms of a (simultaneous) semistable model. In particular, asindicated by the lifting theorem of Amini-Baker-Brugalle-Rabinoff, it encodes all combinatorialand residual algebra-geometric information about f. My talk will be mainly concerned with the morecomplicated case of wild covers, where new discrete invariants appear, with the different function being the most basic one. I will recall its basic properties following my joint work with Cohen and Trushin,and will then pass to the latest results proved jointly with U. Brezner: the different functioncan be refined to an invariant of a residual type, which is a (sort of) meromorphic differential form on the reduction, so that a lifting theorem in the style of ABBR holds for simplest wild covers.

Syntomic regulators for K_2 of curves with arbitrary reduction

Series
Algebra Seminar
Time
Monday, January 29, 2018 - 15:00 for 1 hour (actually 50 minutes)
Location
Skyles006
Speaker
Amnon BesserGeorgia Tech/Ben-Gurion University
I will explain how to explicitly compute the syntomic regulator for varieties over $p$-adic fields, recently developed by Nekovar and Niziol, in terms of Vologodsky integration. The formulas are the same as in the good reduction case that I found almost 20 years ago. The two key ingrediants are the understanding of Vologodsky integration in terms of Coleman integration developed in my work with Zerbes and techniques for understanding the log-syntomic regulators for curves with semi-stable reduction in terms of the smooth locus.

Vologodsky and Coleman integration on curves with semi-stable reduction

Series
Algebra Seminar
Time
Monday, November 27, 2017 - 15:00 for 1 hour (actually 50 minutes)
Location
Skyles006
Speaker
Amnon BesserGeorgia Tech/Ben-Gurion University
Let X be a curve over a p-adic field K with semi-stable reduction and let $\omega$ be a meromorphic differential on X. There are two p-adic integrals one may associated to this data. One is the Vologodsky (abelian, Zarhin, Colmez) integral, which is a global function on the K-points of X defined up to a constant. The other is the collection of Coleman integrals on the subdomains reducing to the various components of the smooth locus. In this talk I will prove the following Theorem, joint with Sarah Zerbes: The Vologodsky integral is given on each subdomain by a Coleman integrals, and these integrals are related by the condition that their differences on the connecting annuli form a harmonic 1-cocyle on the edges of the dual graph of the special fiber.I will further explain the implications to the behavior of the Vologodsky integral on the connecting annuli, which has been observed independently and used, by Stoll, Katz-Rabinoff-Zureick-Brown, in works on global bounds on the number of rational points on curves, and an interesting product on 1-forms used in the proof of the Theorem as well as in work on p-adic height pairings. Time permitting I will explain the motivation for this result, which is relevant for the interesting question of generalizing the result to iterated integrals.

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.

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