UGC seminar

The Utrecht Geometry Center seminar is the biweekly pure mathematics colloquium from Utrecht University. In its current incarnation, talks are mostly given by our own members, explaining their work to the whole department in an approachable manner.

  • UGC talks: They will take place on Tuesdays 16.00-17.00, in person in HFG (for a precise location, see the corresponding announcement email) and online over Teams.
  • MI talks: The MI-talks on pure mathematics will take place on Tuesdays 16.00-17.00, in person in the HFG library and online over Teams.
  • How to attend online: The biweekly invitations to the Teams meetings are sent to the UGC and master students mailing lists. Invitations for the MI-talks are sent to all staff. If you have not received an announcement, please do write to one of the organisers (see below).
  • Videos: Recordings of previous talks can be accessed on our Youtube channel. Links to each individual talk can be found below together with their titles and abstracts.

Upcoming talks:

June 21, 2022

Title: Localization techniques in Enumerative Geometry (MI-talk)

Speaker: Sergej Monavari

A classical way to produce invariants is through Intersection Theory, usually on a smooth projective variety. We give a gentle introduction on how to use torus actions to refine invariants in several directions, in particular in K-theory, and on how to weaken smoothness and properness assumptions. As a concrete example, we explain how to extract meaningful invariants from the moduli space of zero-dimensional quotients of a locally free sheaf on a toric variety, and illustrate various closed formulas for different flavours of “higher rank Donaldson-Thomas invariants of points”, which solve a series of conjectures proposed in String Theory. This is based on joint work with N. Fasola and A. Ricolfi.

June 23, 2022

Title: TBA (applied MI-talk, also at 4pm)

Speaker: Rob Bisseling

TBA

Previous talks:

June 7, 2022

Title: Computing endomorphism rings and Frobenius matrices of Drinfeld modules

Speaker: Mihran Papikian (Pennsylvania State University)

video

Let $mathbb{F}_q[T]$ be the polynomial ring over a finite field $mathbb{F}_q$. We study the endomorphism rings of Drinfeld $mathbb{F}_q[T]$-modules of arbitrary rank over finite fields. We compare the endomorphism rings to their subrings generated by the Frobenius endomorphism and deduce from this a reciprocity law for the division fields of Drinfeld modules. We then use these results to give an efficient algorithm for computing the endomorphism rings and discuss some interesting examples produced by our algorithm. This is joint work with Sumita Garai.

May 24, 2022

Title: Derived Hecke operators (MI-talk)

Speaker: Jack Davies

video

Modular forms are classical objects in number theory, and Hecke operators on these modular forms are a useful organisational tool. In this talk, I will define modular forms over the complex numbers and the classical Hecke operators which act upon them. Then I will discuss a generalisation of this theory from the complex numbers to a theory for general rings, and eventually for derived rings. Finally, I would like to show some little games we can play with these kinds of derived Hecke operators, as a sample of their utility. Only basic complex analysis will be assumed for the first half, and in the second half we will use some language from modern algebraic geometry.

May 10, 2022

Title: Concrete abstract computability theory (MI-talk)

Speaker: Jetze Zoethout

slides and video

In the 1930s, Turing, Kleene, Church and others proposed a precise, mathematical notion of a computable function on the natural numbers. For the first time in history, this allowed mathematicians to prove that certain problems are not solvable by an algorithm. Moreover, it turned out that the computability theory of the natural numbers enjoys certain "algebraic" properties that also manifest themselves elsewhere. A structure satisfying these algebraic properties is called a partial combinatory algebra (PCA), and the study of PCAs can be seen as abstract recursion theory. In this talk, I want to give you a flavor of the subject by considering some concrete examples of PCAs, and how they relate to each other.

April 26, 2022

Title: The blowup formula for instanton Vafa-Witten invariants

Speaker: Nick Kuhn (Max Planck Institute, Bonn)

As a consequence of the S-duality conjecture, Vafa and Witten conjectured certain symmetries concerning invariants derived from spaces of vector bundles on a closed Riemannian four-manifold. We focus on the case of a smooth complex projective surface X, where a satisfying mathematical definition of Vafa-Witten invariants has been given by Tanaka and Thomas. Their invariants are a sum of two parts, one of which can be defined in terms of moduli spaces of stable vector bundles on X. Focusing on this instanton part of the VW invariants one can ask how it changes under a basic algebraic operation: Blowing up the surface X at a point. I will report on joint work with Oliver Leigh and Yuuji Tanaka towards an answer to this question.

April 12, 2022

Title: Introduction to operads (MI-talk)

Speaker: Ieke Moerdijk

Operads were introduced in the 1970s in an attempt to describe the algebraic structure of loop spaces in topology, but have since made their appearance in many other parts of mathematics. The goal of this talk will be to give an introduction to the theory of operads accessible to our master students. If time permits, I will end with a new view on (Koszul) duality for operads.

March 29, 2022

Title: Polarizations of abelian varieties over finite fields via canonical liftings

Speaker: Stefano Marseglia

slides and video

We describe all polarizations for all abelian varieties over a finite field in a fixed isogeny class corresponding to a squarefree Weil polynomial, when one variety in the isogeny class admits a canonical liftings to characteristic zero, i.e., a lifting for which the reduction morphism induces an isomorphism of endomorphism rings.

This is joint work with Jonas Bergström and Valentijn Karemaker.

March 15, 2022

Title: Tame topology and a finiteness theorem for variations of Hodge structures

Speaker: Thomas Grimm

slides and video

About 20 years ago physicists found a remarkable set of solutions to some of the fundamental equations arising from string theory. It has subsequently been conjectured by Douglas and others that this set of solutions is finite. This conjecture has since shaped the view of string theory as a predictive theory leading to a finite number of possible universes.

In this talk, I will first describe the underlying mathematical statement of this conjecture as a finiteness condition on the set of self-dual integral classes in a variation of Hodge structure. I will then argue that the conjecture can be proved using a recent remarkable result of Bakker, Klingler, and Tsimerman that connects Hodge theory and tame topology. I will give a brief introduction to tame topologies built from o-minimal structures and describe the structure that tames the period mapping capturing the variation of a Hodge structure. The finiteness theorem for self-dual classes is a generalization of a theorem for the locus of Hodge classes by Cattani, Deligne, and Kaplan.

This talk is based on joint work with B. Bakker, C. Schnell, and J. Tsimerman, see 2112.06995 [math.AG].

March 1, 2022

Title: In Search of the Absolute: Zeuthen's holism and the enumerative geometry of conics

Speaker: Nicolas Michel

Throughout his long and distinguished career, the Danish mathematician Hieronymous Georg Zeuthen constantly contributed to the emergence of enumerative methods in geometry. He did so alternatively as observer, as participant, as professor, or even as referee in the context of the frequent disputes which plagued the early development of this theory. His 1914 Lehrbuch der abzählenden Methoden der Geometrie, the sum of this prolonged engagement, opens on surprising pronouncements: in lieu of first theorems or definitions, we find a philosophical discussion of the "meaning of numbers" and the "relativity of the concepts of general and particular."

Drawing on Zeuthen's holistic outlook on the history and epistemology of geometry, this talk will set out to explain why, to an algebraic geometer at the onset of the 20th century, answering such technical questions of mathematics might first require philosophical detours—and what we might lose by discarding them.

February 15, 2022

Title: Totally geodesic subvarieties in the Torelli locus

Speaker: Carolina Tamborini

Riemannian symmetric spaces are Riemannian manifolds with special symmetry properties. They are important in various fields of geometry. In the seminar, we will be interested in the following fact: the moduli space A_g of principally polarized abelian varieties is a quotient of the Siegel space, which is a Riemannian symmetric space. This fact can be used to approach the study of the Torelli locus, which is the closure in A_g of the image of the moduli space M_g of smooth, complex algebraic curves of genus g via the Torelli map j: M_g-->A_g.

We will first introduce Riemannian symmetric spaces and their totally geodesic submanifolds. Next, we will describe the problem of studying the geometry of the Torelli locus in A_g and its relation with totally geodesic submanifolds of the Siegel space. Finally, we will explain how this is linked to a famous conjecture from Coleman and Oort.

January 18, 2022

Title: On the cohomology of moduli spaces of stable pointed curves: genus 4 and small n, and the vanishing of H^7 and H^9

Speaker: Carel Faber

After an introduction to moduli spaces of curves, I will discuss joint work with Jonas Bergström and Sam Payne. For n at most 3, we determine the relevant terms of the point count of the moduli space M_{4,n} of smooth n-pointed curves of genus 4 over finite fields (S_n-equivariantly). Using several earlier results, we then determine the cohomology of the moduli space overline{M}_{4,n} of stable n-pointed curves of genus 4 and the full count for M_{4,n} and we deduce the mentioned vanishing for all g and n

November 16, 2021

Title: Where symplectic becomes complex

Speaker: Gil Cavalcanti

video

Generalised complex structures are a simultaneous generalisation of complex and symplectic manifolds, but differently from those two cases, generalised complex structures are not homogeneous and have nontrivial local invariants. In fact, in a connected manifold the structure can be symplectic at some points and complex at other points, that is, its type can change. This type changing behaviour is a rich source of puzzles. I will review the basics of generalised complex geometry, including its applications, then explain the main results that allow us to deal with these type change points and possible consequences of these. This talk is based on joint work with Bailey and Leer-Duran.

November 2, 2021

Title: Lie groups, Whittaker functions and Fourier inversion (MI-talk)

Speaker: Erik van den Ban

video

We will explain certain recent results in harmonic analysis on the class of real reductive Lie groups for the simplest example: the special linear group SL(2,R). In that setting our results are expressible in terms of a spectral decomposition associated with the classical Whittaker ODE.

The talk will not presuppose knowledge of the theory of Lie groups.

October 19, 2021

Title: From K-theory to calculus

Speaker: Niall Taggart

video

Topological K-theory is an extension of algebraic K-theory to the world of geometry. Building from vector spaces (say over the real or complex numbers) Atiyah constructed an invariant of topological spaces which behaves similarly to how algebraic K-theory behaves as an invariant of rings. Depending on one's choice of the base field we obtain different invariants which are intricately related.

Also starting with real or complex vector spaces as the foundations, one can describe a categorification of differential calculus which studies functors from your category of vector spaces to the category of topological spaces. This calculus has much in common with differential calculus including the ability to examine the rate-of-change of a functor and a version of Taylor’s Theorem which provides a filtration of a topological space by “polynomial” parts.

In this talk, I will aim to provide some intuition behind these two seemingly unrelated constructions and discuss how the intricate relationship between real and complex K-theory offers deep insight into the calculus. If time permits I will discuss some other categorifications of calculus and how they relate to each other.

October 5, 2021

Title: Integrality of instanton numbers

Speaker: Frits Beukers

video

In a famous 1995 paper on mirror symmetry Candelas, de la Ossa, Greene and Parkes discovered a remarkable relation between certain numbers (named instanton numbers) arising from a classical family of Calabi-Yau 3-folds and counting the number of rational curves of given degree on a general quintic threefold in projective space. This discovery was driven by arguments from physics, but mathematically it was not known that these numbers are integers for a long time. In this talk I describe some work together with Masha Vlasenko that gives some insight in this integrality.

September 21, 2021

Title: The characters behind the large sieve

Speaker: Lasse Grimmelt

Multiplicative and Additive characters of Z/qZ play an important role in number theory. We will take a look how they interact with the so called large sieve inequality. We show how their basic orthogonality properties are combined with some simple analysis to derive surprising results. These results are foundation to much of our progress in prime number research in the last 70 years. While motivated by this applications, the talk will focus on the role of characters.

June 8, 2021

Title: Kodaira dimension of moduli spaces

Speaker: Martin Moeller (U. Frankfurt). Note: The talk will start at 17:30.

We give a brief overview about the motivation for studying the Kodaira dimension of moduli spaces and summarize results for classical moduli spaces. We then turn attention to moduli spaces of flat surfaces and give recent results about their Kodaira dimension.

June 8, 2021

Title: TBA

Speaker: Kathrin Bringmann (U. Cologne). Note: The talk will start at 16:45.

TBA

June 1, 2021

Title: Canonical decomposition of rational maps

Speaker: Mikhail Hlushchanka

slides and video

There are various classical and more recent decomposition results in mapping class group theory, geometric group theory, and complex dynamics (which include celebrated results by Bill Thurston). The goal of this talk is to introduce a novel powerful decomposition of rational maps based on the topological structure of their Julia sets. Namely, we will discuss the following result: every postcritically-finite rational map with non-empty Fatou set can be canonically decomposed into crochet maps (these have very "thinly connected" Julia sets”) and Sierpinski carpet maps (these have very "heavily connected" Julia sets). If time permits, I will discuss applications of this result in various aspects of geometric group theory. Based on a joint work with Dima Dudko and Dierk Schleicher.

May 18, 2021

Title: Why did Greek geometers construct?

Speaker: Viktor Blasjo

video

Why did Greek mathematicians spend hundreds of years trying to make an angle the third of another, or a cube twice the volume of another, in dozens of different ways? What sin could be so grave that they imposed on themselves such a Sisyphean task? In my view, constructions were a foundational program to ensure consistency, validate diagrammatic reasoning, and protect against hidden assumptions. Pushing this view to its logical conclusion leads to accepting that geometry is based in physical reality, not in abstract thought - a view that is in much better agreement with ancient sources than many commentators, both ancient and modern, have cared to admit. Furthermore, this perspective suggests new interpretations and reconstructions of operationalist aspects of solutions to the classical problems that are missing in surviving sources. Notably that: Archytas's cube duplication was originally a single-motion machine; Diocles's cissoid was originally traced by a linkage device; Greek conic section theory was based on the conic compass, and in a few cases string constructions.

April 20, 2021

Title: Endomorphisms of algebraic groups from the viewpoint of dynamical systems

Speaker: Gunther Cornelissen

slides and video

Consider the following two problems:

(a) count the number of irreducible polynomials over a finite field, asymptotically in thedegree of the polynomial (i.e., a polynomial analogue of the prime number theorem) and
(b) compute the number of invertible nxn matrices over a finite field.

These apparently different looking problems can both be interpreted in tems of dynamics of an endomorphism of an algebraic group (where the endomorphism is “Frobenius” and the algebraic group is the additive group or GL(n), and we count orbits, or fixed points). The talk is about a vast generalisation of these results to arbitrary endomorphisms of arbitrary algebraic groups with finitely many fixed points. The results will be explained by example and by picture.

Typical technical keywords are: adelic distortion of linear recurrent sequences, non-hyperbolicity in dynamical systems, Steinberg’s formula for reductive groups, rationality of Artin-Mazur zeta functions, analogue of the Riemann Hypothesis for orbit counts (ongoing joint work with Jakub Byszewski and Marc Houben).

April 6, 2021

Title: Partial Combinatory Algebras - Variations on a Topos-theoretic Theme

Speaker: Jaap van Oosten

video

One of the cornerstones of Logic is the theory of computable functions. In this talk, we show that a common axiomatics underlies both the set of computable functions and certain spaces of continuous functions. The models of these axioms - partial combinatory algebras - serve as building blocks of certain elementary toposes. In the talk, I shall endeavour to familiarize you with the basic ideas. Towards the end, I will point out some recent work by Jetze Zoethout.

March 23, 2021

Title: Proof of a Magnificent Conjecture

Speaker: Martijn Kool

Solid partitions are piles of boxes in the corner of a 4-dimensional room. Their enumeration is a mystery since MacMahon proposed an incorrect formula around 1916. Motivated by super-Yang-Mills theory on (complex) 4-dimensional affine space, Nekrasov recently assigned a measure to solid partitions and proposed a conjectural formula for their weighted enumeration.

We give a geometric definition of this measure using the Hilbert scheme of points on 4-dimensional affine space. Although this Hilbert scheme is very singular and has “higher obstruction spaces”, we can use recent work of Oh-Thomas to localise our invariants and prove Nekrasov’s conjecture. Joint work with J. V. Rennemo.

March 9, 2021

Title: From exotic spheres to equivariant homotopy

Speaker: Mingcong Zeng

In 1956, Milnor discovered that there are several smooth structures on the 7-spheres that are not diffeomorphic to the standard one. In 1963, Kervaire and Milnor provided a method of counting the number of smooth structures on spheres of dimension greater than 4. In this talk, I will start with their results, and talk about how this geometric problem was transformed into a problem in homotopy theory, and discuss how equivariant homotopy comes into play. Finally, I will talk about some recent results with my collaborators on equivariant homotopy and some confusing problems I am thinking about.

February 23, 2021

Title: Generating sets for symplectic capacities

Speaker: Fabian Ziltener

video

This talk is about joint work with my former Ph.D.-student Dušan Joksimović.

Symplectic geometry originated from classical mechanics, where the canonical symplectic form on phase space appears in Hamilton's equation. It is related to dynamical systems and algebraic geometry, among other fields.

Roughly speaking, a (symplectic) capacity is a real-valued function on the class of all symplectic manifolds, satisfying some natural conditions. The set of all capacities may intuitively be viewed as the dual of the class of all symplectic manifolds. Helmut Hofer et al. posed the following problem: Find a minimal set of capacities that generates all capacities.

The main result presented in this talk is that every such generating set has cardinality bigger than the continuum. This diminishes the hope of finding manageable generating sets of symplectic capacities.

February 9, 2021

Title: Periodic versions of algebraic K-theory

Speaker: Lennart Meier

video

Algebraic K-theory is a fundamental invariant of rings. Originally conceived in algebraic geometry by Grothendieck, today it plays a significant role also in topology and number theory. It was an insight of Thomason that one can define periodic versions of algebraic K-theory. Recent work of Bhatt-Clausen-Mathew and Land, Mathew, Tamme and myself shows that this periodic algebraic K-theory has very nice properties. If time permits, I will also discuss our work on higher periodic versions.

January 26, 2021

Title: Random Vector Functional Link Neural Networks as Universal Approximators (Note: the talk will start at 5:30pm)

Speaker: Palina Salanevich

video

Single layer feedforward neural networks (SLFN) have been widely applied to solve problems such as classification and regression because of their universal approximation capability. At the same time, iterative methods usually used for training SLFN suffer from slow convergence, getting trapped in a local minimum and being sensitivity to the choice of parameters. Random Vector Functional Link Networks (RVFL) is a randomized version of SLFN. In RVFL, the weights from the input layer to hidden layer are selected at random from a suitable domain and kept fixed in the learning stage. This way, only output layers are optimized, which makes learning much easier and cheaper computationally. Igelnik and Pao proved that the RVFL network is a universal approximator for a continuous function on a bounded finite dimensional set. In this talk, we provide a non-asymptotic bound on the approximation error, depending on the number of nodes in the hidden layer, and discuss an extension of the Igelnik and Pao result to the case when data is assumed to lie on a lower dimensional manifold.

January 12, 2021

Title: Brownian motion in Riemannian manifolds

Speaker: Rik Versendaal

video

When studying stochastic processes, and diffusions in particular, one of the most important processes is Brownian motion. Brownian motion is, in a way, the natural analogue of the normal distribution for processes. Furthermore, it is intimately related to the heat equation, since the Laplacian describes the infinitesimal evolution of Brownian motion. We will look at how to define and construct Brownian motion in a Riemannian manifold, the so-called Riemannian Brownian motion. There are various ways to do this, both geometric and probabilistic in nature.

First of all, as mentioned above, we can consider the process generated by the Laplacian of the Riemannian manifold, i.e., the Laplace-Beltrami operator. Second, we can use an invariance principle. In Euclidean space, this states that the paths of suitably scaled random walks converge to Brownian motion. One can define an analogue of random walks in manifolds, so-called geodesic random walks, and use these to obtain Riemannian Brownian motion in the limit.

Finally, Riemannian Brownian motion can also be obtained in a geometric way from a Euclidean Brownian motion. The idea is that we can transfer curves in Euclidean space to a manifold by suitably rolling the manifold along the curve. By Malliavin's transfer principle, it turns out that this also makes sense for stochastic processes. In particular, if we roll the manifold along a Euclidean Brownian motion, we will obtain a Brownian motion in the manifold.

If time permits, we will look into some results regarding large deviations for Riemannian Brownian motion. These large deviations are concerned with quantifying exponentially small probabilities of atypical trajectories of Brownian motion with vanishing variance. In particular, the action of a trajectory determines the exponential rate of decay of the probability. This can be shown to hold even in time-evolving Riemannian manifolds, i.e., manifolds where the metric depends on time.

December 22, 2020

Title: From Poisson Geometry to (almost) geometric structures

Speaker: Marius Crainic

I will report on an approach to general geometric structures (with an eye on integrability) based on groupoids endowed with multiplicative structures; Poisson geometry (with its symplectic groupoids, Hamiltonian theories and Morita equivalences) will provide us with some guiding principles. This allows one to discuss general "almost structures" and an integrability theorem based on Nash-Moser techniques (and this also opens up the way for a general "smooth Cartan-Kahler theorem").

This report is based on collaborations/discussions with Francesco Cataffi (almost structures), Ioan Marcut (Nash-Moser techniques), Maria Amelia Salzar (Pfaffian groupoids).

December 8, 2020

Title: Abelian varieties over finite fields

Speaker: Valentijn Karemaker

video

Abelian varieties are algebro-geometric objects with a rich arithmetic structure. Over finite fields, the rational points on these varieties can be understood through arithmetic invariants like the zeta function. I will introduce these varieties and describe some ways in which they have come up in my work.

November 24, 2020

Title: Solving polynomial equations in many variables in primes

Speaker: Shuntaro Yamagishi

Solving polynomial equations in primes is a fundamental problem in number theory. For example, the twin prime conjecture can be phrased as the statement that the equation x-y-2=0 has infinitely many solutions in primes. In this talk, I will talk about some results related to solving F = 0 in primes, where F is a more general higher degree polynomial. It will be a light introduction to the topic, and knowing what prime numbers and polynomials (in more than one variable) are should be enough to follow most of the talk.

November 10, 2020

Title: Rational points on Fano varieties

Speaker: Marta Pieropan

Fano varieties form one of the fundamental classes of building blocks in the birational classification of algebraic varieties. In this talk I will discuss how their special geometric properties can be used to study their arithmetic. I will focus on a few conjectures about their rational points over number fields (potential density, Manin’s conjecture) and how they can be investigated by determining the asymptotic behavior of certain counting functions of rational points.

All the main objects involved will be introduced and illustrated by examples. A summary of the literature on the topic will be discussed, including my own contribution.

October 27, 2020

Title: Counting quaternion algebras, with applications to spectral geometry

Speaker: Lola Thompson

We will introduce some classical techniques from analytic number theory and show how they can be used to count quaternion algebras over number fields subject to various constraints. Because of the correspondence between maximal subfields of quaternion algebras and geodesics on arithmetic hyperbolic manifolds, these counts can be used to produce quantitative results in spectral geometry. This talk is based on joint work with B. Linowitz, D. B. McReynolds, and P. Pollack.

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