If you would like to give a talk, please email one of us!
Fridays, 3:00–4:00pm, Pearce Hall, Room 108.
Please note changed room this semester
Date  Speaker  Title 
Sep 7  Jordan Watts (CMU)  An Introduction to Smootheology 
Sep 14  Anirban Dawn (CMU)  An abstract Fejér theorem and some applications 
Sep 20 2:30 to 3:30 pm Note special day and time Place: Pearce 307 
Nikita Nikolaev (Université de Genève)  Abelianisation of Logarithmic $\mathfrak{sl}(2)$Connections 
Sep 21  Rasul Shafikov (UWO)  On rationally convex embeddings and immersions of real manifolds in complex spaces 
Sep 28  Yoav Len (Univ of Georgia)  Lifting Tropical Intersections 
Oct 5  David Jensen (Univ of Kentucky)  Linear Systems on General Curves of Fixed Gonality 
Oct 12  Jordan Watts (CMU)  Differential Forms on Orbit Spaces 
Oct 19  Nicola Tarasca (Rutgers University)  $K$classes of BrillNoether varieties and a determinantal formula 
Oct 26  James Heffers (UMichAnn Arbor)  Geometric Properties of Upper Level Sets of Lelong Numbers of Currents on $\mathbb{P}^2$ 
Nov 9  Alexander Izzo (BGSU)  Polynomial hulls and analytic structure 
Nov 16  Matthew Satriano (Univ of Waterloo)  Interpolating Between the BatyrevManin and Malle Conjectures 
Nov 30  Martino Fassina(UIUC)  Type conditions for real hypersurfaces in $\mathbb{C}^n$ 
Dec 7  Nathan Priddis (Brigham Young University)  BHK Mirror symmetry and beyond 
Jan 25 2pm, Pearce 138 Note special time and place 
Karen Smith (Univ of Michigan) AMS Grad Student Chapter event 
Resolution of Singularities 
Feb 8  Harry Richman (Univ of Michigan)  Distribution of tropical Weierstrass points 
Feb 15  Trieu Le (Univ of Toledo)  Algebraic properties of $m$isometries 
Feb 22  Neha Prabhu (Queen's Univ, Canada)  Fluctuations in the distribution of Hecke eigenvalues 
Mar 1  Mohit Bansil (MSU)  Spectral Theory of Kohn Laplacians on Spheres 
Mar 15  Zhenghui Huo (Univ of Toledo)  Weighted $L^p$ norm estimate for the Bergman projection on the Hartogs triangle. 
Mar 29  Luke Edholm (Univ of Michigan)  Derivatives of Integral operators and Mapping Properties. 
Apr 12  Seth Wolbert (Univ of Manitoba, Canada)  Short exact sequences of Lie group(oid)s, weak actions, and presentations of actions on stacks 
Apr 19 4:15 pm Note special time 
Yuan Zhang (PurdueFort Wayne)  A rigidity phenomenon for CR submanifolds 
Apr 26  Dmitry Zakharov (CMU)  The failure of the localtoglobal principle for cubic equations 
Speaker: Jordan Watts
Title: Introduction to Smootheology
Abstract:
Smooth manifolds form a successful theory in geometry that uses topology to generalize real analysis from Euclidean space to much more general shapes. For example, think of how the 2torus locally looks like a disk in the plane at every point. This is exactly what a manifold is: a space that locally looks like a disk/ball in Euclidean space. However, in practice, mathematics is full of singular spaces, where singularities spoil the local structure, but we still need to do analysis at this locations. Think of the apex of the double cone $z^2 = x^2+y^2$ in 3space.
In this talk, I will introduce you to a few attempts to generalize smooth manifolds to bigger categories in order to include singular and function spaces, and mention some of their pros and cons.
Speaker: Anirban Dawn
Title: An Abstract Fejér theorem and some applications
Abstract: A famous theorem of Fejér (1899) states that if $f$ is a continuous function on the unit circle $\mathbb{T}$, then the Cesàro means of the partial sums of the Fourier series of $f$ converge to the function $f$ uniformly.
Du BoisReymond produced an example of a continuous function on $\mathbb{T}$ where the partial sums do not converge to the function, even pointwise. In this talk we will give a generalization of Fejér's result. Let $\mathbb{T}^n$
be the $n$dimensional unit torus, $X$ be a quasicomplete locally convex topological vector space and $\sigma$ be a continuous representation of $\mathbb{T}^n$ on $X$. For $x \in X$, we define the Fourier series of $x$ with respect to the representation
$\sigma$ and we show that the Cesàro means of the partial sums of the Fourier series converge to $x$ in the topology of $X$. After that we give some applications of this theorem to Fourier series and spaces of holomorphic functions.
Speaker: Nikita Nikolaev
Title: Abelianisation of Logarithmic $\mathfrak{sl}(2)$Connections
Abstract: I will describe an approach to studying meromorphic connections on vector bundles called abelianisation.
This technique has its origins in the works of FockGoncharov (2006) and GaiottoMooreNeitzke (2013), as well as the WKB analysis.
Its essence is to put rank$n$ connections on a complex curve $X$ in correspondence with much simpler objects:
connections on line bundles over an $n$fold cover $\Sigma\to X$. The point of view is similar in spirit to abelianisation of Higgs bundles, aka the spectral correspondence: Higgs bundles on $X$
are put in correspondence with rankone Higgs line bundles on a spectral cover $\Sigma\to X$.
However, unlike Higgs bundles, abelianisation of connections requires the introduction of a new object, which we call the Voros cocycle.
The Voros cocycle is a cohomological way to encode objects such as ideal triangulations that appeared in FockGoncharov, spectral networks
that appeared in GaiottoMooreNeitzke, as well as the connection matrices appearing in the WKB analysis.
By focusing our attention on the simplest case of logarithmic singularities with generic residues,
I will describe an equivalence of categories, which I call the abelianisation functor, between $\mathfrak{sl}(2)$connections on $X$
satisfying a certain transversality condition and rankone connections on an appropriate 2fold spectral cover $\Sigma \to X$.
This presentation is based on the work completed in my thesis (2018) and recent extensions thereof.
Speaker: Rasul Shafikov
Title: On rationally convex embeddings and immersions of real
manifolds in complex spaces.
Abstract: A classical result of DuvalSibony characterizes rationally
convex totally real embeddings of real manifolds into $\mathbb C^n$
as those that are isotropic with respect to some Kahler form. In this
talk I will describe some generalizations of this result for topological
embeddings and immersions, and will discuss some applications.
Speaker: Yoav Len
Title: Lifting Tropical Intersections
Abstract: My talk is concerned with combinatorial aspects of intersection theory. When tropicalizing algebraic varieties, each of their intersection points maps to a tropical intersection point. Characterizing this locus is a fundamental problem in tropical ge ometry. In my talk, I will appeal to nonArchimedean and poly hedral geometry to characterize the locus in various cases. The solution leads to a combinatorial tool for counting multitangent hyperplanes of algebraic varieties, detecting dual defects, and for computing Newton polygons of dual varieties. I will not assume any knowledge in tropical geometry.
Speaker: David Jensen
Title: Linear Systems on General Curves of Fixed Gonality
Abstract: The geometry of an algebraic curve is governed by its linear systems. While many curves exhibit bizarre and pathological linear systems, the general curve does not. This is a consequence of the BrillNoether theorem, which says that the space of linear systems of given degree and rank on a general curve has dimension equal to its expected dimension. In this talk, we will discuss a generalization of this theorem to general curves of fixed gonality. To prove this result, we use tropical and combinatorial methods. This is joint work with Dhruv Ranganathan, based on prior work of Nathan Pflueger.
Speaker: Jordan Watts
Title: Differential Forms on Orbit Spaces
Abstract: Differential forms are mathematical constructs used in a wide variety of contexts: they are what you integrate in calculus, they provide extra structure to spaces such as curvature or a symplectic structure, and these in turn are used to describe physical systems. Moreover, differential forms are important for obtaining topological invariants, such as de Rham cohomology.
In this talk, we will discuss differential forms in the context of symmetry: a famous result of Koszul states that the singular cohomology of an orbit space coming from a compact Lie group action is the same as the cohomology of the basic differential forms (forms that respect the symmetry). In fact, if the action is free (trivial stabilisers), then the de Rham complex of forms on the orbit space is isomorphic to the complex of basic differential forms.
Using the theory of diffeology, this relationship between differential forms extends to the nonfree case; that is, we allow the orbit space to have singularities. Consequently, we have a triple of isomorphic cohomology theories: that from basic differential forms, singular cohomology on the orbit space, and diffeological de Rham cohomology on the orbit space.
Speaker: Nicola Tarasca
Title: $K$classes of BrillNoether varieties and a determinantal formula
Abstract: BrillNoether varieties for pointed curves parametrize linear series on curves with prescribed vanishing at marked points. I will present a formula for the Euler characteristic of the structure sheaf of BrillNoether varieties for curves with at most two marked points. The formula recovers the classical Castelnuovo number in the zerodimensional case, and previous work of EisenbudHarris, Pirola, ChanLópezPfluegerTeixidor in the onedimensional case. The result follows from a new determinantal formula for the Ktheory class of certain degeneracy loci of maps of flag bundles. This is joint work with Dave Anderson and Linda Chen.
Speaker: James Heffers
Title: Geometric Properties of Upper Level Sets of Lelong Numbers of Currents on $\mathbb{P}^2$
Abstract:
Lelong numbers are a useful tool for complex analysts wanting to look at the mass a current $T$ has at a given point. In this talk we look at the geometric properties of sets of points where a current $T$ has ``large" Lelong numbers, and see that the points where our current has large Lelong number can be contained in a small subspace of $\mathbb{P}^2$. The talk will start with introductory definitions and some simple examples, then look at the following result. Let $T$ be a positive closed current of bidegree $(1,1)$ with unit mass on the complex projective space $\mathbb P^2$. For $\alpha > 2/5$ and $\beta = (22\alpha)/3$ it has been shown that if $T$ has four points with Lelong number at least $\alpha$, the upper level set $E_{\beta}^+ (T)$ of points of $T$ with Lelong number strictly larger than $\beta$ is contained within a conic with the exception of at most one point. We will investigate some examples showing the sharpness of the assumptions of this results as well as look into some interesting components of its proof.
Speaker: Alexander Izzo
Title: Polynomial Hulls and Analytic structure
Abstract:
It was once hoped that whenever a compact set in complex Euclidean space has a nontrivial polynomial hull, the hull must contain an analytic disc. This hope was shattered by a counterexample given by Stolzenberg in 1963. Over the 55 years since then, several additional constructions of hulls without analytic discs have been found. Nevertheless, the issue of analytic structure in polynomial hulls is still not well understood. I will present new results regarding (the absence of) analytic structure in polynomial hulls.
Speaker: Matthew Satriano
Title: Interpolating Between the BatyrevManin and Malle Conjectures
Abstract: The BatyrevManin conjecture gives a prediction for the asymptotic growth rate of rational points on varieties over number fields when we order the points by height. The Malle conjecture predicts the asymptotic growth rate for number fields of degree d when they are ordered by discriminant. The two conjectures have the same form and it is natural to ask if they are, in fact, one and the same. We develop a theory of point counts on stacks and give a conjecture for their growth rate which specializes to the two aforementioned conjectures. This is joint work with Jordan Ellenberg and David ZureickBrown.
Speaker: Martino Fassina
Title: Type conditions for real hypersurfaces in $\mathbb{C}^n$
Abstract: To every point of a real hypersurface in complex space, one can attach an invariant which measures the maximum order of contact of the hypersurface with complex $q$dimensional varieties.
This number is known as the $q$type and was first introduced by D'Angelo. Since real hypersurfaces are the boundaries of complex domains, their geometry has deep interactions with the function theory of $\mathbb{C}^n$.
In particular, work of Kohn and Catlin has related the $q$type to the local boundary regularity of the CauchyRiemann equations.
In this talk I will compare different ways of measuring the $q$type of a real hypersurface that appear in the literature.
I will then show how this work can be exploited to obtain quantitative information on the regularity of some complex PDEs.
Speaker: Nathan Priddis
Title: BHK Mirror symmetry and beyond
Abstract:
I will discuss a phenomenon from physics called Mirror symmetry, and a particularly nice version of mirror symmetry called BHK mirror symmetry after BerglundHuebsch and Krawitz. Then I will discuss two variations on BerglundHuebsch mirror symmetry for BorceaVoisin models, and for nonabelian LandauGinzburg models.
Speaker: Karen Smith
Title: Resolution of Singularities
Abstract:
Algebraic varieties are geometric objects defined
by polynomials – you have known many examples
since high school, where you learned that a circle
can be defined by a polynomial equation such as
$x^2+y^2=1$. Polynomials can define incredibly
complicated shapes, such as a mechanical arm in
medical software or Woody’s arm in Toy Story,
but yet they can be easily manipulated by hand or
by computer. For this practical reason, algebraic
geometry – the study of algebraic varieties and the
equations that define them – is a central research
area within modern mathematics. It is also one of
the oldest and most beautiful. In this talk, I hope
to share my love of the subject, which stems from
the way the geometry and algebra interact,
including some open problems and my favorite
tools for attacking them.
Speaker: Harry Richman
Title: Distribution of tropical Weierstrass points
Abstract:
The set of (higher) Weierstrass points on a metric graph of genus g > 1 is an analogue of the set of Ntorsion points on a circle. As N grows, the torsion points "distribute evenly" over a circle. This makes it natural to ask how Weierstrass points distribute on a graph, as the degree of the corresponding divisor grows. We will explore how Weierstrass points behave on metric graphs (i.e. tropical curves) in analogy with Riemann surfaces (i.e. complex algebraic curves), and explain how their distribution can be described in terms of electrical networks. Knowledge of tropical curves will not be assumed, but knowledge of how to compute resistances (e.g. in series and parallel) will be useful.
Speaker: Trieu Le
Title: Algebraic properties of $m$isometries
Abstract: $m$Isometries are Hilbert space bounded linear operators satisfying an operator equation that generalizes the notion of isometries. These operators were introduced by Agler in the early nineties. A few years ago, Bermudez, Martinon, and Noda proved that the sum of an isometry and a commuting nilpotent operator of order $s$ is a $(2s1)$isometry. This interesting result shows that any Jordan block whose entries on the main diagonal have modulus one is an $m$isometry for an appropriate value of $m$. Their proof, which makes use of combinatorial arguments, is quite lengthy and complicated. In this talk, I will discuss my approach which not only provides an elegant proof of the above result but also allows us to obtain more general results.
Speaker: Neha Prabhu
Title: Fluctuations in the distribution of Hecke eigenvalues
Abstract: A famous conjecture of Sato and Tate (now a celebrated theorem of Taylor et al) predicts that the normalised pth Fourier coefficients of a typical Hecke eigenform follow the semicircle distribution as the primes p grow. On averaging over families of Hecke eigenforms, we study the distribution of the deviations about the average value, and obtain a central limit theorem. This is joint work with Kaneenika Sinha.
Speaker: Mohit Bansil
Title: Spectral Theory of Kohn Laplacians on Spheres
Abstract: A CRmanifold is a submanifold in $\mathbb{C}^M$ with extra structure stipulating that the dimension of the complex part of its tangent space is pointwise invariant under some complex structure map. The Kohn Laplacian $\Box_b$ is a second order differential operator intrinsically defined on any CRmanifold whose spectrum reveals important geometric information.
In the case of the Rossi sphere $(\mathbb{S}^3, \mathcal{L}^t)$ showing that 0 is in the essential spectrum of $\Box_b$ is enough to conclude that $(\mathbb{S}^3, \mathcal{L}^t)$ is not embeddable into $\mathbb{C}^M$ for any $M > 0$. One approach is to study the Kohn Laplacian on the subspaces of spherical harmonics in $L^2(\mathbb{S}^{2N1})$. When restricted to finite dimensional subspaces of spherical harmonics, $\Box_b$ can be expressed as a matrix and one can either explicitly compute or at least obtain bounds on the eigenvalues. Furthermore, when the CR structure is induced from the ambient manifold $\mathbb{C}^M$ (unperturbed operator), the spectrum of $\Box_b$ is explicitly computed on any sphere $\mathbb{S}^{2N1}\subset\mathbb{C}^M$
by Folland.
In this project we expand on the previous work and study asymptotics of the spectrum on the Rossi sphere, and we provide upper and lower bounds on the maximum eigenvalues on the invariant subspaces. In the unperturbed case we examine the eigenvalue counting function and obtain asymptotics analogous to Weyl’s law.
Speaker: Zhenghui Huo
Title: Weighted $L^p$ norm estimate for the Bergman projection on the Hartogs triangle.
Abstract:
The boundedness of the Bergman projection on the weighted $L^p$ space of the unit ball was first studied by Bekollé and Bonami in the 80s. Using modern dyadic harmonic analysis techniques, sharp weighted $L^p$ estimates were obtained for the Bergman projection on the upper half plane by Pott and Reguera in 2012, and on the unit ball in $\mathbb C^n$ by Rahm, Tchoundja, and Wick in 2016. In this talk, I will introduce the dyadic operator technique used for these results and give a sharp weighted $L^p$ norm estimate for the Bergman projection on the Hartogs triangle. This work is joint with Brett Wick.
Speaker: Luke Edholm
Title: Derivatives of Integral operators and Mapping Properties.
Abstract: The Bergman projection on the ball in $\mathbb{C}^n$ is known to extend to a bounded operator on the $L^p$Sobolev spaces for the full range $p\in(1, \infty)$ Similar results hold for many classes of smoothly bounded, pseudoconvex domains. In 2016, Edholm and McNeal showed a limited $L^p$ mapping range for the Bergman projection on a class of domains called generalized Hartogs triangles, but derivatives were not considered in this work. In this talk, we show that the study of Sobolev mapping properties of the Bergman projection on these domains (and any other Reinhardt domain) requires analysis of new kernels which are not conjugate symmetric. This leads to many surprising consequences. We give a complete answer to the $L^p$Sobolev mapping properties of the Bergman projection on the generalized Hartogs triangles.
Speaker: Seth Wolbert
TitleShort exact sequences of Lie group(oid)s, weak actions, and presentations of actions on stacks
Abstract: It is a well known result from algebra that, given a short exact sequence of Lie groups $1\rightarrow K \rightarrow G \rightarrow H \rightarrow 1$, we can express $G$ as a semidirect product associated to an action of $H$ on $K$ exactly when there exists a splitting (i.e., a smooth homomorphism section) $\sigma:H\to G$. Of course, such a section may fail to exist. However, in the case where a smooth section $\sigma$ which is not a homomorphism exists, $H$ admits what might be called a weak action on $K$: an "action" which fails the axioms of a group action in a controlled way. More generally, one may regard $G$ as (a presentation of) an action of $H$ on $K$ regarded as stacks. In my talk, I will discuss these previously known results as well as my work to extend these results to yield an equivalence between Lie groupoid fibrations (the analogous object to a short exact sequence in the world of Lie groupoids) and actions of a Lie groupoid on a stack.
Speaker: Yuan Zhang
Title: A rigidity phenomenon for CR submanifolds
Abstract:
In Projective Geometry, it is a classical result that submanifolds of $\mathbf P^N$ with vanishing second fundamental forms or with degenerate Gauss maps are necessarily totally geodesic. In this talk, we discuss some similar phenomenon of CR submanifolds in CR geometry.
Speaker: Dmitry Zakharov
Title:
The failure of the localtoglobal principle for cubic equations
Abstract:
Consider the differential equation $f'(z)=g(z)$ for an unknown function $f$ on an open set $U$ in the complex plane. This equation can be solved as a formal power series at any point of $U$. However,
these local solutions can be patched together into a global solution on all of U only if U is simply connected, in other words, only if $H^1(U)=0.$
We can ask a very similar question in number theory. Suppose that a polynomial equation $f(x,y,z)=0$ with integer coefficients has a solution in every completion of the rational numbers, in other words, in the real numbers and in the padic numbers for all primes $p$. Does it follow that the equation has a rational solution? A theorem of Hasse and Minkowski states that this is true if f is a quadratic polynomial, so we say that quadratic equations satisfy the localglobal principle. In my lecture, I will describe an example, due to Selmer, of a cubic polynomial f for which the localglobal principle fails.
Date  Speaker  Title  
Sep 1  Debraj Chakrabarti (CMU)  Bergman spaces on Reinhardt Domains  
Sep 8  Anirban Dawn (CMU)  Partitions of Unity  
Sep 22  Dmitry Zakharov (CMU)  Divisors on graphs  
Sep 29  Dmitry Zakharov (CMU)  Tropical BrillNoether theory  
Oct 6  Ilya Kachkovsky (Michigan State University)  Band edges of 2D periodic Schrodinger operators  
Oct 13  Sid Graham (CMU)  The Prime Number Theorem  
Oct 20  Martin Ulirsch (University of Michiagn, Ann Arbor)  Tropical geometry of the Hodge bundle  
Oct 27  Mythily Ramaswamy (TIFR, India)  Control of PDE models  
Nov 3  Tim Reynhout (CMU)  Partition of Unity for Symplectic Volumes of Ribbon Graph Complexes.  
Nov 10  Sivaram Narayan  Complex Symmetric Composition Operators on the Hardy Space  
Nov 15  Chaya Norton (Concordia University)  Differentials with real periods and the geometry of M_g SPECIAL DATE AND TIME: 2 pm at PE 223 

Nov 17  Anthony Vasaturo (University of Toledo)  Carleson measures and Douglas' question on the Bergman space on the disk  
Dec 1  Luke Edholm (Univ of Michigan, Ann Arbor)  The Leray Operator on Two Dimensional Model Domains  
Dec 8  Adam Coffman (IUPU Fort Wayne)  An Example for Green's Theorem with Discontinuous Partial Derivatives  
Jan 19  Felix Janda (Univ of Michigan, Ann Arbor)  Moduli of meromorphic functions on algebraic curves  
Feb 9  Nathan Grieve (Michigan State University)  On complexity of rational points and arithmetic of linear series  
Feb 16  Anirban Dawn (CMU)  A Theorem of Grothendieck  
Feb 23  Tanuj Gupta (CMU)  Hörmander's theorem for the CauchyRiemann operator: the onevariable case  
Mar 30  Zeljko Cuckovic (University of Toledo)  $L^p$ Regularity of Bergman Projections on Domains in $\mathbb{C}^n$  
Apr 13  Matthew Woolf (UIC)  Stable Cohomology of Moduli Spaces of Sheaves on Surfaces  
Apr 20  Steven Rayan (University of Saskatchewan)  Asymptotic geometry of hyperpolygons  
Apr 27  Eric Bucher (MSU)  Introducing cluster algebras and their applications 
Speaker:Debraj Chakrabarti
Title: Bergman spaces on Reinhardt domains
Abstract: Let $\Omega$ be a possibly nonsmooth Reinhardt domain in $\mathbb{C}^n$ , and let $A^p(\Omega)$ be the Banach space of holomorphic functions on $\Omega$ whose $p$th powers are integrable, $p\geq 1$. We study properties of $A^p(\Omega)$ as a linear space, for example, the question of convergence of Laurent series of functions in $A^p(\Omega)$
in the norm of $A^p(\Omega)$, and that of determining the dual of $A^p(\Omega)$. These questions have unsurprising answers when $\Omega$ is the unit disc in the plane. We show there are new phenomena in the general situation, some only partially understood. In particular we look at the special case of the Hartog's triangle, where some of the computations can be performed explicitly. This is joint work with Luke Edholm and Jeff McNeal.
Speaker:Anirban Dawn
Title: Partitions of Unity (Expository Talk)
Abstract: In this talk I will introduce a very interesting and well known topic of Differential Topology known as "Partitions of Unity". A partition of unity on a differential manifold $\mathcal{M}$ is a collection of $\mathcal{C}^{\infty}$ (smooth) functions $\{\phi_i : i \in \textit{I} \}$ on $\mathcal{M}$, where $\textit{I}$ is an arbitrary index set, not assumed to be countable, such that the collection of supports $\{\text{supp}(\phi_i) : i\in \textit{I} \}$ is locally finite. Moreover, for any point $p \in \mathcal{M}$ we have $\phi_i(p) \geq 0$ and $\sum_{i \in \textit{I}} \phi_i(p) = 1$. In my talk I will prove the existence of partitions of unity of $\mathcal{M}$ subordinate to an open cover $\{\mathcal{U}_{\alpha} : \alpha \in \Lambda\}$. The proof will need some ideas of topics from point set topology, for example refinement of a cover, local finiteness, paracompactness, which will also be discussed. After that I plan go over the construction of a $\mathcal{C}^{\infty}$ function with compact support on $\mathcal{M}$ which I need to prove the existence. At the end, I will try to talk about some applications of partitions of unity.
Speaker:Dmitry Zakharaov
Title: Divisors on graphs
Abstract:
It has long been understood in combinatorics that there is a remarkable similarity between graphs and algebraic curves, also known as Riemann surfaces. In the last decade or so, it was recognized that this relationship is not a coincidence: graphs are onedimensional algebraic varieties from the point of view of tropical mathematics.
In the first talk, I will introduce divisor theory on graphs. A divisor on a graph is a linear combination of the vertices with integer coefficients. We define an equivalence relationship on divisors, using socalled chipfiring moves, which is similar to linear equivalence of divisors on curves. We can then define the standard objects of curve theory — meromorphic functions and their divisors, the complete linear system associated to a divisor, the genus of a graph, the canonical class — for graphs, and prove analogues of theorems such as the Riemann—Roch theorem and Clifford's theorem. This talk will be elementary and will not use any concept more advanced than that of an abelian group.
Speaker:Dmitry Zakharaov
Title: Tropical BrillNoether theory
Abstract:
In the second talk, I will explain the relationship between the theories of divisors on graphs and on algebraic curves. Given a family of smooth algebraic curves degenerating to a singular curve, the intersections between the irreducible components of the singular curve define a graph, called the dual graph of the family. Baker’s specialization lemma then establishes a relationship between the divisor theory on the degenerating family and on the dual graph. This relationship enables us to reduce algebrogeometric questions, such as the existence or nonexistence of line bundles with prescribed properties, to combinatorial problems on graphs, which are often more tractable. Conversely, it is possible to establish new results in graph theory using known results in algebraic geometry.
Speaker:Ilya Kachkovskiy
Title: Band edges of 2D periodic Schrodinger operators.
Abstract:
: The bandgap structure of the spectra of periodic Schrodinger operators can be described in terms of spectral band functions which manifest dispersion relations between the energy and the quasimomentum. It is widely believed that, by a small perturbation of the potential, one can make all the extrema of these functions nondegenerate, i. e. isolated and having nonvanishing Hessians. This is also equivalent to the notion of effective mass being well defined for generic potentials. We establish the “isolated” part for all sufficiently regular periodic potentials, without the need of a perturbation.
The talk is based on the joint work with Nikolay Filonov.
Speaker:Sid Graham
Title: The Prime Number Theorem.
Abstract:
For a positive real number $x$, let $\pi(x)$ denote the number of primes that are less than or equal to $x$.
The Prime Number Theorem (PNT) states that $\pi(x)$ is asymptotically $x/\log x$.
This was first proved independently by Hadamard and de la Vall\'ee Poussin in 1896.
In 1899, de la Vall\'ee Poussin gave a version of PNT with an explicit error term.
In this talk, I will sketch a proof of the Prime Number Theorem. It turns out that PNT is equivalent
to showing that the Riemannzeta function has no zeros with real part 1. The primary tools in the
proof are the Hadamard Factorization Theorem and the Functional Equation of the Riemannzeta function.
Speaker: Martin Ulirsch
Title: Tropical geometry of the Hodge bundle
Abstract: The Hodge bundle is a vector bundle over the moduli space of curves whose fiber over a smooth curve (of genus g) is the space of abelian differentials on this curve. We may define a tropical analogue of its projectivization as the moduli space of pairs (\Gamma, D) consisting of a stable tropical curve \Gamma and an effective divisor D in the canonical linear system on \Gamma. This tropical Hodge bundle turns out to have dimension 5g5, while it is a classical fact that the projective Hodge bundle has dimension 4g4. This means that not every pair (\Gamma, D) in the tropical Hodge bundle arises as the tropicalization of a suitable element in the algebraic Hodge bundle.
In this talk I am going outline a comprehensive (and completely combinatorial) solution to the realizability problem, which asks us to determine the locus of points in the tropical Hodge bundle that arise as tropicalizations. Our approach is based on recent work of BainbridgeChenGendronGrushevskyMöller on compactifcations of strata of abelian differentials. Along the way, I will also develop a modulitheoretic framework to understand the specialization of divisors to tropical curves as a natural tropicalization map in the sense of AbramovichCaporasoPayne.
This talk is based on joint work with Bo Lin, as well as on an ongoing project with Martin Möller and Annette Werner.
Speaker:Mythily Ramaswamy
Title: Control of PDE models (Colloquium Talk)
Abstract: Starting with a brief introduction to
control of ODE systems, I will discuss similar
issues for PDE systems.
The focus will be on linear viscoelastic fluid flow
models, a system of
coupled partial differential equations for velocity
and stress.
Speaker: Tim Reynhout
Title: Partition of Unity for Symplectic Volumes of Ribbon Graph Complexes.
Abstract: As an example of Anirban's talk on partitions of unity, we will briefly develop the idea of ribbon graph complexes and the tools necessary to create the partition of unity used for calculating their volume. Ribbon graphs are combinatorially defined objects which can be viewed as graphs on open or closed Riemann surfaces. This allows for results to be extended to the moduli space of Riemann Surfaces.
Speaker:Sivaram Narayan
Title: Complex Symmetric Composition Operators on the Hardy Space
Abstract: We say that a bounded operator $T$ on a complex Hilbert space $H$ is complex symmetric if there exists a conjugation (i.e., a conjugate linear, isometric involution) $J$ such that $T=JT^*J$. In this talk, we will first discuss a few general results about complex symmetric operators on a Hilbert space. We will then focus for most of the talk on the complex symmetry of composition operators $C_\varphi f=f\circ \varphi$ induced on the Hardy space $H^2$ by analytic selfmaps $\varphi$ of the open unit disk $\mathbb{D}$. We show that there are complex symmetric composition operators on $H^2$ induced by $\varphi$ that are linearfractional but not automorphisms. In doing so, we answer a recent question of Noor, and partially answer the original problem posed by Garcia and Hammond. This is a joint work with Sievewright and Thompson. We will briefly mention the work in progress (with Sievewright and Tjani) on the weighted Hardy spaces.
Speaker: Chaya Norton
Title: Differentials with real periods and the geometry of $M_g$
Abstract: A differential on a Riemann surface is called real normalized (RN) if the result of integrating the differential along any closed loop is real. Alternatively the imaginary part of the antiderivative of the differential is a welldefined harmonic function on the Riemann surface, and in this form RN differentials have been studied by Maxwell. In the 80s Krichever introduced RN differentials in the development of the spectral theory of the nonstationary Schrodinger operator.
The moduli space M_g is roughly the collection of genus g Riemann surfaces up to isomorphism. Relatively recently Grushevsky and Krichever have shown that RN differentials provide a useful perspective to study the geometry of M_g by noticing that for any fixed singular part of a differential at marked points, there exists a unique RN differential. We will introduce these objects and attempt to outline the perspective it provides on the geometry of M_g and vanishing tautological classes. In joint work with Grushevsky and Krichever we developed the degeneration theory for RN differentials.
Speaker:Anthony Vasaturo
Title: Carleson measures and Douglas' question on the Bergman space on the disk
Abstract: Motivated by Douglas' question about the invertibility of Toeplitz operators on the Hardy Space, we study a related question concerning the Berezin transform and averaging function of a Carleson measure for the weighted Bergman space of the disc. As a consequence, we obtain a necessary and sufficient condition for the invertibility of Toeplitz operators whose symbols are averaging functions of these Carleson measures.
Speaker: Luke Edholm
Title: The Leray Operator on Two Dimensional Model Domains
Abstract: One major difference between complex analysis in one and several variables is the lack of a true analogue to the onevariable Cauchy transform, $\mathbf{C}$. However, by looking at domains satisfying a convexity condition, we are able to construct the Leray transform, $\mathbb{L}$, which shares many of $\mathbf{C}$'s familiar properties. A significant amount of recent work has been done to study the mapping properties of $\mathbb{L}$ in various settings. This talk will focus on a family of model domains in $\mathbb{C}^2$, and discuss new techinques used in the analysis of the Leray operator. These models can be used to locally approximate a very general class of domains, and it is expected that the theorems in the model case will carry over to the general case. I will also discuss what these results mean in terms of dual CR structures on hypersurfaces in projective space. This is joint work with Dave Barrett.
Speaker:Adam Coffman
Title: An Example for Green's Theorem with Discontinuous Partial Derivatives
Abstract: Green’s Theorem in multivariable calculus is usually stated with a hypothesis that the partial derivatives are continuous. I will present an example of a function where the partial derivatives exist but are discontinuous, to which a stronger version of Green’s Theorem applies.
Speaker:Felix Janda
Title: Moduli of meromorphic functions on algebraic curves
Abstract: I will discuss three different ways to compactify (the class of) the
locus of meromorphic functions on algebraic curves with prescribed
zeros and poles. One of them is called the double ramification cycle.
Speaker:Nathan Grieve
Title: On complexity of rational points and arithmetic of linear series
Abstract:
I will survey recent results which pertain to diophantine and arithmetic aspects of linear series on projective varieties. One theme is that complexity of rational points should be measured on rational curves. These results are consequences of Schmidt's Subspace Theorem. Further, I will explain how these theorems can be interpreted using ideas from toric geometry. For example, I will indicate connections to the theories of Chow forms and Okounkov bodies.
Speaker:Anirban Dawn
Title: A Theorem of Grothendieck.
Abstract: It is an important problem in analysis to find the duals of function spaces. One of these is the Fréchet space $\mathcal{O}(G)$, the space of all holomorphic functions on an open set $G \subset \mathbb{C}$. Following work by da Silva Dias and Köthe, it was shown by Grothendieck (1953) that $\mathcal{O}(G)^{*}$, the dual of $\mathcal{O}(G)$, is naturally isomorphic to $\mathcal{O}_{0}(\hat{\mathbb{C}} \setminus G)$, the space of holomorphic functions in a neighborhood of $\hat{\mathbb{C}} \setminus G$ which vanish at $\infty \in \hat{\mathbb{C}}$. We will prove the result for the special case when $G$ is the open unit disc and will describe the main ideas of the proof in the general case.
Speaker:Tanuj Gupta
Title: Hörmander's theorem for the CauchyRiemann operator: the onevariable case.
Abstract: Many problems in complex analysis can be reduced to solving the inhomogeneous CauchyRiemann equations, which is referred to as a $\overline{\partial}$problem.
In 1965, L. Hörmander proved a remarkable estimate for the $\overline{\partial}$problem in $L^2$ norms. In this talk we discuss the statement and proof of this result in the case of
domains in the complex plane.
Speaker:Zeljko Cuckovic
Title: $L^p$ Regularity of Bergman Projections on Domains in $\mathbb{C}^n$
Abstract:
Bergman projections and Bergman kernels are among the central objects in complex analysis. In this talk we will discuss the $L^p$ regularity of weighted Bergman projections on various domains in $\mathbb{C}^n$. Then we will show an $L^p$ irregularity of weighted Bergman projections on complete Reinhardt domains with exponentially decaying weights (joint work with Yunus Zeytuncu). Finally we establish estimates of the $L^p$ norms of Bergman projections on strongly pseudoconvex domains.
Speaker:Matthew Woolf
Title: Stable Cohomology of Moduli Spaces of Sheaves on Surfaces
Abstract:
On any algebraic surface, there is a moduli space of torsionfree stable sheaves with fixed rank and Chern classes. In general, these moduli spaces can be very badly behaved. However, if you fix the rank and determinant and let the second Chern class go to infinity, things change. In this case, the moduli spaces become nonempty and irreducible of the expected dimension, and the singular locus has arbitrarily large codimension.
In this talk, I will discuss work with Izzet Coskun that suggests these spaces are in fact converging, at least homotopically. Specifically, we will show that for certain surfaces, the Betti numbers of these moduli spaces stabilize. Moreover, there is a simple formula for the stable Betti numbers which is independent of rank and first Chern class. If time permits, we will discuss how to think of this as a variation of the AtiyahJones conjecture.
Speaker:Steven Rayan
Title: Asymptotic geometry of hyperpolygons
Abstract:
Nakajima quiver varieties lie at the interface of geometry and representation theory and provide an important class of examples of CalabiYau manifolds. I will discuss a particular instance, hyperpolygon space, which arises from a certain shape of quiver. The simplest of these is a noncompact complex surface admitting the structure of an "instanton", and therefore fits nicely into the KronheimerNakajima classification of ALE hyperkaehler 4manifolds, which is a geometric realization of the McKay correspondence for finite subgroups of SU(2). For more general hyperpolygon spaces, we can speculate on how this classification might be extended by studying the geometry of hyperpolygons at "infinity". This talk represents previous work with Jonathan Fisher and ongoing work with Hartmut Weiss.
Speaker:Eric Bucher
Title: Introducing cluster algebras and their applications
Abstract:
Cluster algebras were first invented by Fomin and Zelevinsky in 2003 to study total positivity of canonical bases. Since their inception, these mathematical objects have popped up in a large variety of seemingly unrelated areas including: Teichmuller theory, CalabiYao categories, integrable systems, coordinate rings of Grassmannians and the study of high energy particle physics. In this talk we will lay the basic groundwork for working with cluster algebras as well as discuss a few of their applications to the above areas. This talk is intended to be introductory so no background or definitions will be assumed. The intention is to have everyone walk away having learned about this new and fascinating mathematical object.