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Gjesteforelesninger og seminarer

Kommende

Simulation of the Richtmeyer–Meshkov instability
Tid og sted: 28. sep. 2021 10:1512:00, Seminar room 1020, Niels Henrik Abel's house

The PDE seminars for the Autumn of 2021 will be held every Tuesday from 10:15–12:00

Tid og sted: 30. sep. 2021 14:0015:45, NHA B1120

In a famous paper, Geir Ellingsrud and Stein Arild Strømme use the Atiyah-Bott localization theorem in equivariant cohomology to compute the number of complex twisted cubics on a complete intersection. Motivated by results from A1-homotopy theory there is a new way of doing such enumerative counts which works over an arbitrary base field, not only the complex numbers. Recently, Marc Levine proved a version of Atiyah-Bott localization for this new way of counting.

In the talk I will recall the classical Atiyah-Bott localization theorem and explain how one can use it in enumerative geometry. Furthermore, I will explain how this new way of counting works and present some results about twisted cubics on complete intersections counted this way. This is based on joint work with Marc Levine.

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Tid og sted: 1. okt. 2021 11:0012:00, Online
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Tid og sted: 5. okt. 2021 14:1516:00, Erling Sverdrups plass, Niels Henrik Abels hus, 8th floor

Marginal maximum likelihood estimation of longitudinal latent variable models for ordinal observed variables is challenging due to the high latent dimensionality required to accurately model residual dependencies for repeated measurements. We use second-order Laplace approximations to the high-dimensional integrals in the marginal likelihood function for longitudinal item response theory models and implement an efficient estimation method based on the approximations. The method is illustrated with items from the Montreal Cognitive Assessment, administered at four time points in a Hong Kong study of aging and well-being. We discuss the limitations of the proposed estimation method and outline a potential extension to the approach that uses a dimension-reduction technique.

Tid og sted: 7. okt. 2021 14:1516:00, NHA B1120

Stable polynomials are a multivariate generalization of real-rooted univariate polynomials. This notion of stability for hypersurfaces can be extended to lower-dimensional varieties, giving rise to positively hyperbolic varieties. I will present results showing that tropicalizations of positively hyperbolic varieties are very special polyhedral complexes with a rich combinatorial structure. This, in particular, generalizes a result of P. Brändén showing that the support of a stable polynomial must be an M-convex set.

Tid og sted: 14. okt. 2021 14:1516:00, NHA B1120
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Tid og sted: 15. okt. 2021 11:0012:00, Online
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Tid og sted: 18. okt. 2021 14:1516:00, Erling Sverdrups plass, Niels Henrik Abels hus, 8th floor

In this talk I will discuss the variational form of Bayes theorem by Zellner (1988). This result is the rationale behind the variational (approximate) inference scheme, although it is not always that clear in modern presentations. I will discuss two applications of this results. First, I will show how to do a low-rank mean correction within the INLA framework (with amazing results), which is essential for the next generation of the R-INLA software currently in development. In the second one, I will introduce the Bayesian   learning rule, which unify many machine-learning algorithms from fields such as optimization, deep learning, and graphical models. This includes classical algorithms such as ridge regression, Newton's method, and Kalman filter, as well as modern deep-learning algorithms such as stochastic-gradient descent, RMSprop, and Dropout.

The first part of the talk is based on our recent research at KAUST, while the second part is based upon \texttt{arxiv.org/abs/2107.04562} with Dr. Mohammad Emtiyaz Khan, RIKEN Center for AI Project, Tokyo.

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Tid og sted: 19. okt. 2021 14:1516:00, Erling Sverdrups plass, Niels Henrik Abels hus, 8th floor
Tid og sted: 21. okt. 2021 14:1516:00, NHA B1120
Tid og sted: 4. nov. 2021 14:1516:00, NHA B1120

Tidligere

Tid: 23. sep. 2021 14:1516:00

For the second talk, I will talk about how to relate relative Gromov--Witten invariants with relative periods via relative mirror symmetry and, given a degeneration, how relative periods and (absolute) periods are related on the mirror side.  

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Tid og sted: 17. sep. 2021 11:0012:00, Online
Tid og sted: 16. sep. 2021 14:1516:00, NHA B1120
For the first talk, I will talk about the story of Gromov--Witten theory on the A-side. Relative Gromov--Witten invariants play a central role in computing Gromov--Witten invariants via the degeneration formula. I will give a summary of some recent progress of basic structures in relative Gromov--Witten theory.
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Tid og sted: 14. sep. 2021 14:1516:00, Erling Sverdrups plass, Niels Henrik Abels hus, 8th floor

Template Model Builder (TMB) is an R-package that is well suited for performing fast inference with latent Gaussian models when the likelihood can be written as a three times differentiable function. TMB automatically differentiates the likelihood and utilizes Markov structures to efficiently integrate over latent variables with the Laplace approximation. In this seminar I will first introduce TMB and then elaborate a fish stock assessment model implemented with use of TMB that provides quota advice for approximately 25 different fish species in Europe.

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Tid og sted: 9. sep. 2021 14:1516:00, NHA B1120

A cohomology class of a smooth complex variety of dimension n has coniveau ≥c if it vanishes in the complement of a closed subvariety of codimension ≥c, and has strong coniveau ≥c if it comes by proper pushforward from the cohomology of a smooth variety of dimension ≤n−c. We show that these two notions differ in general, both for integral classes on smooth projective varieties and for rational classes on smooth open varieties. This is joint work with Olivier Benoist.

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Tid og sted: 3. sep. 2021 11:0012:00, Online
Et bilde av Kristian Ranestad
Tid og sted: 2. sep. 2021 14:1516:00, NHA B1120

A graded Artinian Gorenstein ring A is a quotient of a polynomial ring S with the apolar ideal of a homogeneous form. The Betti numbers of the resolution of A as an S-module are invariants to the homogeneous form. In joint work with Michal and Gregorz Kapustka, Hal Schenck, Mike Stillman and Beihui Yuan, we use these Betti numbers to describe a stratification of the space of quartics in four variables.

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Tid og sted: 2. sep. 2021 10:1512:00, NHA B1120

Alberto Merici will present his paper (joint with Frederico Binda and Shuji Saito) "Derived log Albanese sheaves".

Tid og sted: 31. aug. 2021 10:1511:15, NHA B1119

Oliver Röndigs will give a one hour talk entitled "Homotopy of SLn and the relation between Milnor- and Quillen K-theory"

Tid og sted: 27. aug. 2021 12:1513:00, Niels Henrik Abels hus, 9th floor, seminar room 919

Abstract: It is known that a sessile drop subject to a forced vibration will vibrate in different shapes depending on the frequency of the forcing, the drop’s liquid properties and the liquid/solid/gas system. So the question then becomes, what can these vibrating drops help us understand? Here we find that we can use the motion of these drops to understand the constitutive law relating the drop’s apparent dynamic contact angle to its contact line velocity. We find we are able to extract mobility parameters like those described by the Davis-Hocking model, and that mobility parameters extracted in this fashion can be used in simulations of drop-drop coalescence to accurately predict post-coalescence dynamics.

This talk is part of the Mechanics Lunch Seminar series. That means 20min talks plus discussion in an informal setting and bring-your-own-lunch.

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Tid og sted: 20. aug. 2021 11:0012:00, Online
Tid og sted: 25. juni 2021 12:1513:00, Zoom

Abstract: Efficient and parameter robust solvers for multiscale/multiphysics systems, where the coupling is enforced by the Lagrange multipliers, rely on operators in fractional Sobolev spaces defined over the interface. Arguably, this feature is not unexpected as there is explicit coupling/an interfacial variable in the system. However, in this talk we show that even for coupled problems free of Lagrange multipliers the fractional operators are a crucial component for constructing robust preconditioners. Stokes-Darcy/Biot systems will be discussed.

This talk is part of the Mechanics Lunch Seminar series. That means 20min talks plus discussion in an informal setting.

Zoom: To obtain the Zoom meeting details please contact Timo Koch (timokoch at math.uio.no).

Tid og sted: 18. juni 2021 12:1513:00, Zoom

Abstract: New estimates of heat loss from Earth's interior (the mantle) suggest that the Pacific side of the planet has been losing heat at a much higher rate compared to the African side. The difference in heat loss amounts to almost 50 degrees more cooling of the Pacific side over the past 400 million years.

This talk is part of the Mechanics Lunch Seminar series. That means 20min talks plus discussion in an informal setting.

Zoom: To obtain the Zoom meeting details please contact Timo Koch (timokoch at math.uio.no).

Tid og sted: 11. juni 2021 12:1513:00, Zoom

Abstract: Magmatic intrusions into the earth's crust lead to geological formations such as sills, laccholites and volcanoes. The conventional approach is to threat the intrusive fluid as Newtonian and viscous, while the host-rock is assumed to behave purely elastic. However, Magma is known to have non-Newtonian properties. In addition, field studies indicate that visco-elastic deformation of the host rock is an important effect in the propagating fracturing. We thus want to investigate the effect of introducing a yield-stress fluid in the intrusion process. We perform an experiment based on the elasto-hydro-dynamical approach, but where we change the injected fluid from a viscous (glycerol) to a yield-stress fluid (carbopol). We are interested in seeing how this potentially can change the dynamics of the intrusion compared with the viscous case. 

This talk is part of the Mechanics Lunch Seminar series. That means 20min talks plus discussion in an informal setting.

Zoom: To obtain the Zoom meeting details please contact Timo Koch (timokoch at math.uio.no).