Math Calendar

Tuesday, November 29, 2022
16:00-17:00
HFG 611
MI talks
Dirk Siersma - Polar degree of projective hypersurfaces in the presence of singularities
For any hypersurface V in projective n-space P, given by f=0, the notion of polar degree is defined as the topological degree of the (projectivized) gradient mapping of the homogeneous polynomial f. This is a map from P-V to P.

We will discuss first the history of polar degree and give several examples, e.g. the determinant hypersurface has polar degree 1. The hypersurfaces with polar degree are called homaloidal and are of extra interest because the gradient map is bi-rational.

Polar degree zero is related to the question of what happens if the Hessian of f is identically zero. This was solved by Gordan and Noether in 1876.

After a long period of algebraic studies, recently topological methods gave some interesting results. Dolgacev classified in 2000 all the projective homoloidal plane curves: a short list. Huh determined in 2014 all homoloidal hypersurfaces in P with at most isolated singularities.  

In this talk we will reprove Huh's results with methods of singularity theory. Moreover we will prove the Huh's conjecture that his list of polar degree 2 surfaces with isolated singularities is complete!

Finally we say something more about hypersurfaces with the non-isolated singularities.


Website of the UGC seminar (and the pure side of the MI-alks): http://utrechtgeometrycentre.nl/ugc-seminar/  Practical details will be sent by email to the staff and student mailing lists a few days before the event.
Wednesday, November 30, 2022
11:00-12:00
HFG-409
Algebraic Geometry Talk
Navid Nabijou - Roots and logs in the enumerative forest
Logarithmic and orbifold structures provide two different paths to the enumeration of curves with fixed tangencies to a normal crossings divisor. Simple examples demonstrate that the resulting systems of invariants differ, but a more structural explanation of this defect has remained elusive. I will discuss joint work with Luca Battistella and Dhruv Ranganathan, in which we identify birational invariance as the key property distinguishing the two theories. The logarithmic theory is stable under strata blowups of the target, while the orbifold theory is not. By identifying a suitable system of “slope-sensitive” blowups, we define a “limit" orbifold theory and prove that it coincides with the logarithmic theory. Our proof hinges on a technique – rank reduction – for reducing questions about normal crossings divisors to questions about smooth divisors, where the situation is much-better understood.
 
Webpage of the AG seminar: https://webspace.science.uu.nl/~piero001/index_AG_seminar.html
Thursday, December 1, 2022
16:00-17:00
HFG 611
MI TALK
Palina Salanevich
Title: Phase retrieval with time-frequency structured measurements

Abstract: Phase retrieval is the non-convex inverse problem of signal reconstruction from intensity measurements with respect to a measurement frame. This problem is motivated by practical applications, such as diffraction imaging and audio processing. The nature of the measurements in a particular application determines the structure of the measurement frame. This makes the study of the phase retrieval with structured, application relevant frames especially interesting. 
In the talk, we are going to focus on phase retrieval with Gabor frames, where the measurement vectors follow time-frequency structure that naturally appears in imaging and acoustics applications. We will discuss how to achieve stable and efficient reconstruction with such measurements and how prior information about the signal class can be used to regularize the phase retrieval problem and reduce the number of measurements required for reconstruction. 
Friday, December 2, 2022
11:00-12:45
Minnaert 015
Mark Kac seminar in mathematical physics and probability
Ivan Kryven (UU)
Title: Generation and analysis of sparse random graphs with a given degree sequence


Abstract: For a given graphical degree sequence, there is typically a large number of simple graphs that satisfy it. We will consider the problem of uniformly sampling a graph from this set. This can also be viewed as a model for a random graph, i.e. a graph-valued random variable. In general, algorithms that realise such sampling have exponential complexity, but there are special classes of graphs where the complexity is less severe and could even be linear. In this talk, we will discuss several of such classes called sparse graphs and show that their uniform generation can be achieved in linear time using the so called sequential construction — non-uniformly placing one edge at a time in the course of m steps while updating the probabilities after each step. In the second part of the talk we will discuss the underlying random graph model and show that it gives rise to solutions of interesting nonlinear partial differential equations and can even be regarded as a method for solving such equations formally
14:15-16:00
Minnaert 015
Mark Kac seminar in mathematical physics an probability
Rik Versendaal (UU)
Title: Random graph models with spatial and degree constraints


Abstract:

In this talk we consider random graph models with both spatial information (random geometric graphs) and a prescribed degree sequence (configuration model). When these constraints are considered separately, the random graph models are well understood. However, when imposed together, conflicts arise.

In the first part of this talk, we will consider an algorithm for sampling random graphs with spatial and degree constraints. In particular, we will consider a target degree sequence dn and edge-length distribution fn. Provided that dn is bounded uniformly and fn is not too large compared to the empirical distribution of the available edge-lengths, we will show that our algorithm randomly produces a graph with degree sequence dn and empirical edge-length distribution close to fn.

In the second part, we will look into the emergence of a giant component in these random graph models. We make a first step into this direction by considering a d-dimensional torus partitioned into compartments forming a cubic lattice. We distribute vertices equally over these compartments, and only allow local edges inside compartments and between neighbouring compartments. We assume the number of compartments diverges when the number of vertices grows. Using connections with multitype branching processes, we will prove that if the num- ber of vertices per compartment grows quickly enough, then a giant component emerges under similar conditions on the degree sequence as for the standard configuration model.

All is based on work together with Ivan Kryven. The first part is based on [KV22], while the second part is based on [KV21].

[KV21] Ivan Kryven and Rik Versendaal. “Giant component in the configu- ration model under geometric constraints”. In: ArXiv preprint (2021). url: https://arxiv.org/abs/2108.04112.

[KV22] Ivan Kryven and Rik Versendaal. “Sequential construction of spatial networks with arbitrary degree sequence and edge length distribution”. In: ArXiv preprint (2022). url: https : / / arxiv . org / abs / 2207 . 08527.

Tuesday, December 6, 2022
15:30-18:00
Nijmegen HG00.308
TopICS
Felix Wierstra - A recognition principle for iterated suspensions as coalgebras over the little cubes operad

Felix Wierstra (UvA) 

Tile: A recognition principle for iterated suspensions as coalgebras over the little cubes operad

Abstract: In this talk I will discuss and prove a recognition principle for iterated suspensions as coalgebras over the little disks operad. This is based on joint work with Oisín Flynn-Connolly and José Moreno-Fernández.

For general information on the seminar series, and to subscribe to the mailing list, please consult the seminar webpage: https://sites.google.com/view/nialltaggartmath/seminars/topics.

Thursday, December 8, 2022
11:00-12:00
Minn-3.12
Guest-lecture
Prof.dr. Joo Ho Choi (Korea Aerospace University) - Bayesian methods for uncertainty quantification

Abstract:Bayesian Methods for Uncertainty Quantification

This presentation discusses Bayesian theory andits applications in the field of reliability engineering. It addresses thebasic theory, philosophy, and simulation techniques to compute the posteriordistribution such as Markov Chain Monte Carlo and Particle Filtering methods.Application examples are the life prediction of Lithium-ion batteries, bladesin turbine engine, computer model calibration with uncertainty using real data,and crack growth prediction of aircraft structures.

16:00-17:00
HFG 6.11
Applied Mathematics Seminar -- Yves van Gennip (TUDelft)
Saturday, December 10, 2022
09:25-09:30
Afspraakbevestiging
Thursday, December 15, 2022
16:00-17:00
KBG Pangea
Kan lectures
Luc Illusie (Paris) - Lectures on the de Rham Complex I
First lecture: A brief historical survey

The talk will be followed by drinks in the HFG library.



More info on the Kan memorial lectures: https://utrechtgeometrycentre.nl/daniel-kan-memorial-lectures/
Friday, December 16, 2022
11:00-12:00
KBG Pangea
Kan lectures
Luc Illusie (Paris) - Lectures on the de Rham Complex II
Second lecture: De Rham complexes in mixed characteristic I

More info on the Kan memorial lectures: https://utrechtgeometrycentre.nl/daniel-kan-memorial-lectures/
14:00-15:00
KBG Pangea
Kan lectures
Luc Illusie (Paris) - Lectures on the de Rham Complex III
Third lecture: De Rham complexes in mixed characteristic II

More info on the Kan memorial lectures: https://utrechtgeometrycentre.nl/daniel-kan-memorial-lectures/
Wednesday, January 11, 2023
11:00-12:00
HFG-409
Algebraic Geometry Talk
Lars Halvard Halle - TBA
TBA
Thursday, January 12, 2023
16:00-17:00
HFG 611
MI talk Martin Bootsma

Title: Modeling the impact of Corona measures

AbstractIn this talk, I will discuss a model we have developed for the ministry of Health, Welfare and Sports to estimate the impact of Corona ticket measures (QR-code) on the spread of SARS-CoV-2 in the Netherlands. I will discuss how the underlying assumptions lead to the model formulation and I will briefly discuss some results.

 

Tuesday, January 17, 2023
16:00-17:00
HFG611
UGC talks
Dusan Joksimovic - TBA
TBA

Website of the UGC seminar (and the pure side of the MI-alks): http://utrechtgeometrycentre.nl/ugc-seminar/  Practical details will be sent by email to the staff and student mailing lists a few days before the event.
Friday, January 20, 2023
14:00-16:00
HFG610
Friday Fish
Dusan Joksimovic - TBA
Thursday, January 26, 2023
16:00-17:00
HFG 6.11
MI talk
Cristian Spitoni
Friday, January 27, 2023
14:00-16:00
HFG610
Friday Fish
Yann Guggisberg - TBA
TBA
Tuesday, February 7, 2023
16:00-17:00
HFG611
MI talks
Paige North - TBA
TBA

Website of the UGC seminar (and the pure side of the MI-alks): http://utrechtgeometrycentre.nl/ugc-seminar/  Practical details will be sent by email to the staff and student mailing lists a few days before the event.
Thursday, February 9, 2023
16:00-17:00
HFG 611
Applied talk
Bruno Kimura (U Hokkaido Japan)
Thursday, February 23, 2023
16:00-17:00
HFG 611
Applied seminar talk
Richard Gill (UL)