Events

Upcoming

Time and place: Oct. 18, 2021 12:00 PM1:00 PM, Niels Henrik Abels hus, 12th floor, Abels utsikt

Abstract: The concept of symmetry breaking is well-known in physics, for instance in condensed matter, where it results from interactions in a many-body system — e.g., phase transition in a spin system. Yet, as physicists, we tend not to think of the patterned structures seen in living, many-body systems in terms of broken symmetries. Whether it is the spacing of knuckles on our hand, the collective alignment of hairs on an insect wing, or more globally the transformation of a homogeneous, isotropic embryo into a developed organism, symmetry breaking abounds in biology. What new insights can a physicist bring to understand the origin of these complex phenomena? (Click title to read full abstract below...)

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Time and place: Oct. 18, 2021 2:15 PM4:00 PM, 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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Time and place: Oct. 19, 2021 2:15 PM4:00 PM, Erling Sverdrups plass, Niels Henrik Abels hus, 8th floor

The talk is elementary and discusses empirical modelling of single variables with insurance losses as example. There are in such cases little or no theory to go on, and the amount of data is many situation quite scarce. Why do we so often limit ourselves to fit two-parameter families? It will be suggested that it may be a good idea to work with more flexible models with three or four parameters and that this may provide a nice framework for automating the entire procedure for the computer to work alone. Sure, with little data the parameters may be unstably estimated, but that may not apply equally to the distributions they define. Many-parameter families suitable for insurance losses will be reviewed with some simple asymptotics in an example allowing this and with Monte Carlo to throw light on the issue in other cases.

Time and place: Oct. 21, 2021 12:30 PM1:30 PM, Rest area, 10th floor, NH Abels Hus
Time and place: Oct. 21, 2021 2:15 PM4:00 PM, NHA B1120
Time and place: Oct. 22, 2021 12:15 PM1:00 PM, Niels Henrik Abels hus, 9th floor, seminar room 919

Abstract: Mixed-dimensional partial differential equations (PDEs) are equations coupling unknown fields defined over domains of differing topological dimensions. Such mixed-dimensional PDEs naturally arise in a wide range of fields including geology, biomedicine, and fracture mechanics. We introduce an automated framework dedicated to mixed-dimensional problems as part of the FEniCS library. This talk gives an overview of the abstractions and algorithms involved. The introduced tools will be illustrated by concrete examples of applications in biomedicine (see below for more detailed context).

This talk is part of the Mechanics Lunch Seminar series. Bring-your-own-lunch and lots of questions.

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Time and place: Oct. 25, 2021 10:00 AMOct. 28, 2021 12:00 PM, Zoom

Professor Dan Crisan, Imperial College London, author of several books on filtering is now holding an intensive course.

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Time and place: Oct. 28, 2021 2:30 PM3:30 PM, Rest area, 10th floor, NH Abels Hus
Picture of the candidate.
Time and place: Oct. 29, 2021 1:15 PM, Room 720, Niels Henrik Abels hus, Zoom

Doctoral candidate Qinghua Liu at the Department of Mathematics, Faculty of Mathematics and Natural Sciences, is  defending the thesis Bayesian Preference Learning with the Mallows Model for the degree of Philosophiae Doctor.

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Time and place: Nov. 2, 2021 2:15 PMOct. 19, 2021 4:00 PM, Erling Sverdrups plass, Niels Henrik Abels hus, 8th floor

Cards are drawn, one at a time, with replacement, from a deck of n cards. I study the total time W_n needed until we have seen all n cards, via different perspectives, along with a Gumbel limiting distribution. Various non-trivial identities, involving different perspectives for moments and Laplace transformations, are found as corollaries. These findings are also used to estimate the number of different cards,if uknown. If I needed to sample 133 words from a document, before I had 50 different words, what is the vocabulary size for the document? How many words did Shakespeare know (including those he never used in his writing)? 

An Abels Tårn podcast about some of these themes, which attracted a fair amount of inspired comments and guesses from the public (specifically, finding the mean of W_n above, for the case of n = 52 cards), can be found on the Abels Tårn website, July 2021, as a conversation with Torkild Jemterud, Jo Røislien, and myself. 

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Time and place: Nov. 5, 2021 10:00 AM12:00 PM, Online
Time and place: Nov. 5, 2021 10:15 AM11:00 AM, NHA B1120

First of two lectures on A-infinity structures.

Time and place: Nov. 8, 2021 10:15 AM11:00 AM, NHA B1119

Second of two lectures on A-infinity structures.

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Time and place: Nov. 9, 2021 1:15 PM2:00 PM, Online
Time and place: Nov. 11, 2021 2:15 PM4:00 PM, NHA B1120
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Time and place: Nov. 12, 2021 9:00 AM4:00 PM, Zoom

To keep up the attention high in stochastics and its applications, we organise 1 day workshop on line. Welcome!

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Time and place: Nov. 25, 2021 8:00 AMNov. 26, 2021 3:00 PM, University of Oslo

For the third year in a row, STORM and TMS organize a workshop on rough path theory and related topics. It is held on 25-26 November 2021. 

TMF logo
Time and place: Dec. 6, 2021Dec. 10, 2021, Blindern campus

Winter school at University of Oslo

Nils Henrik on a sunny day
Time and place: Sep. 12, 2022Sep. 14, 2022, The Norwegian Academy of Science and Letters

A conference in honour of Nils Henrik Risebro on the occasion of his 60th birthday.

Previous

Time and place: Oct. 15, 2021 12:15 PM1:00 PM, Niels Henrik Abels hus, 9th floor, seminar room 919

Abstract: Elimination of substances from the brain is believed to occur by a combination of convection and diffusion. In previous work, transport along perivascular spaces around blood vessels have been explicitly meshed and modeled, and also 1D-3D models have been used to model the interaction between blood and brain tissue. A problem with both these approaches is that it requires spatial information of all blood vessels within the brain and in addition may result in a computationally expensive system to solve. In this talk, I will introduce a homogenized model of transport in the brain, also taking into account transfer between different compartments (like blood and brain tissue) within the brain. Fluid flow in and between compartments are modeled with the mulitple-porosity elasticity theory (MPET), while transport within and between compartments are modeled with convection-diffusion equations. I will further show preliminary results from our model and compare with experimental data obtained in a glioma (brain tumor) patient, where transport between blood and brain is typically altered.

This talk is part of the Mechanics Lunch Seminar series. Bring-your-own-lunch and lots of questions.

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Time and place: Oct. 15, 2021 11:00 AM12:00 PM, Online
Time and place: Oct. 14, 2021 2:15 PM4:00 PM, NHA B1120
Planar polypols - “polygons with curved sides” - were proposed by Eugene Wachspress as generalized algebraic finite elements. In order to define barycentric coordinates for polypols, he introduced the adjoint curve of a rational polypol. In recent work by physicists, positive geometries are defined as certain semialgebraic sets together with a meromorphic differential form called the canonical form. We show that a rational regular polypol gives a positive geometry and give an explicit expression for its canonical form in terms of the adjoint and boundary curves of the polypol. In the special case that the polypol is a convex polygon, we show that the adjoint curve is hyperbolic and describe its nested ovals. 
 
This talk is based on joint work with K. Kohn, K. Ranestad, F. Rydell, B. Shapiro, R. Sinn,  M.-S. Sorea, and S. Telen.
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Time and place: Oct. 12, 2021 9:30 AM4:30 PM, Abels utsikt

Now it's time for the traditional Section 3 PhDs' & postdocs' gathering, which will take place in Abels Utsikt on October 12th, 2021, 09.30 - 16.30. All PhD students and postdocs have the opportunity to give a 15-minute talk on their research. In addition, Andrey Pilipenko (National Academy of Sciences of Ukraine/Igor Sikorsky Kyiv Polytechnic Institute) will give a talk on solving ODEs with non-Lipschitz coefficients by perturbation and Hao Tang (UiO) will introduce his research on stochastic fluid models. Welcome!

Time and place: Oct. 8, 2021 12:15 PM1:00 PM, Niels Henrik Abels hus, 9th floor, seminar room 919

Abstract: My soft matter research group investigates the autonomous transformation of phospholipid agglomerates into membrane compartments through a sequence of topological changes on solid interfaces. This process is initiated by contact and wetting of artificially created as well as natural surfaces by the lipids, and proceeds via a network of interconnected lipid nanotubes to produce nearly uniform lipid bilayer compartments. Under minimal assumptions it is conceivable that such process could have occurred on the early Earth, where the autonomous formation of simple membrane compartments is presumed to have enabled encapsulation of nucleotides and prebiotic chemistry precursors. According to the currently accepted “bulk hypothesis”, such compartments have spontaneously formed under moderate environmental conditions from lipids suspended in bulk aqueous medium. Only very recently, surfaces have emerged as potential supporting structures for the self-assembly of prebiotic compartments. In my talk, I will report on new evidence for the involvement of surfaces in protocell nucleation and development. The talk will highlight the implications of the new findings for our understanding of possible origin of life processes, and argue that materials properties-driven autonomous processes on solid interfaces might have greater role in the development of life than currently considered.

Time and place: Oct. 7, 2021 2:15 PM4:00 PM, 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.