Tidlegare gjesteforelesninger og seminarer - Side 12

Rubinur Khatun, Postdoctoral Fellow at Institute of Theoretical Astrophysics, University of Oslo.

QOMBINE seminar by Snorre Bergan (UiO)

By Ludovic Orlando, University of Toulouse, France (Notice the time!)

Atul Mohan, Postdoctoral Fellow at Rosseland Centre for Solar Physics, University of Oslo.

The Section 4 seminar for the Autumn of 2022 will be held on Thursdays from 10:15–12:00 (see the schedule)

By Craig R. Primmer from the University of Helsinki, Finland

Thore Espedal Moe, PhD fellow of Rosseland Centre for Solar Physics (RoCS), University of Oslo.

Consider the singularity C^4/(Z/2), where Z/2 acts as the matrix diag(-1,-1,-1,-1). This singularity is special, in that it does not admit a crepant resolution. However, it does admit a so-called noncommutative crepant resolution, given by a Calabi-Yau 4 quiver. The moduli space of representations of this quiver turns out to share a lot of similarities with moduli spaces of sheaves over Calabi-Yau fourfolds, and it turns out that we can reuse techniques from studying moduli of sheaves to define and compute invariants of this moduli space of representations. In this talk, I will explain how these invariants can be defined, and give conjectures about the forms of these invariants. This talk is based on joint work with Raf Bocklandt.

The Thoralf Skolem Memorial Lecture 2022

by prof. Holger von Wenckstern

Department of Physics, UiO

Øyvind Christiansen, PhD student at Institute of Theoretical Astrophysics, University of Oslo.

Specialization of (stable) birational types is an important tool when studying (stable) rationality in families. A crucial ingredient is to cook up one parameter degenerations such that the limit has certain combinatorial and geometric properties. Nicaise-Ottem studied these questions for hypersurfaces in algebraic tori, and used tropical geometry to construct degenerations that would have been hard (impossible) to construct geometrically. Even after these are constructed one must carefully study the limit in order to apply specialization techniques, this involves both combinatorics and questions about variation of stable birational types. I will talk about the specialization technique in the setup of Nicaise-Ottem, explain some natural questions that appear through the combinatorics, and give some positive results in this direction.

QOMBINE seminar talk by David Jaklitsch (Hamburg)

Ingeborg Gjerde (Simula Research Laboratory) presents joint work with Ridgway Scott (University of Chicago).

Abstract: Airflow around airplane wings is characterized by a wide range of flow scales, making it highly challenging to capture numerically. From a simulation viewpoint, the following questions are still being actively investigated: Why do airplanes fly? Can one reliably simulate the lift and drag of an airplane wing? In this talk, I will provide no good answers to these questions. Instead, I want to talk about some interesting results I've stumbled into tangentially, including:
- (Nonlinear) kinetic energy instability analysis, also referred to as Reynolds-Orr instability
- Slip boundary conditions and their connection to D'Alembert's paradox
- Stokes' paradox and its connection to weighted Sobolev spaces. I will show numerical results computed for flow around a cylinder, which serves as a proxy for flow around an airplane wing. In particular, I will talk about the impact of the friction boundary condition on the drag force and flow stability. Finally, I will comment on how these results might be interpreted in view of: New Theory of Flight, J. Hoffman, J. Jansson, C. Johnson (2016), Journal of Mathematical Fluid Mechanics.

Lars Frogner, PhD fellow of Rosseland Centre for Solar Physics (RoCS), University of Oslo.

The variety of sums of powers, VSP(F, r) of a homogeneous form F of rank r is the closure in the Hilbert scheme of apolar schemes of length r. A bad limit is a scheme in the closure that is not apolar to F. I will discuss examples of bad limits, including examples for quadrics found by Joachim Jelisiejew that contradicts earlier results on polar simplicies. This is report on work in progress with Jelisiejew and Schreyer and with Grzegorz and Michal Kapustka.

Maksym Brilenkov, PhD student at Institute of Theoretical Astrophysics, University of Oslo.

Counterexamples to the integral Hodge conjecture can arise either from torsion cohomology classes (as in Atiyah's and Hirzebruch's original counterexample from 1961) or from non-torsion classes (as first seen in Kollár's counterexample from 1991). After Voisin proved the IHC for uniruled threefolds, Schreieder found a unirational fourfold where the IHC fails. His construction of a non-algebraic Hodge class relies on abstract arguments with unramified cohomology. It was an open question whether this class is of torsion type. In this talk, I want to explain a new method that gives an explicit geometric description of the unramified cohomology class appearing in his argument. In particular, this approach allows to prove that Schreieder's unirational counterexample is of torsion type.