Seminaria

, 605
Ergodyczne twierdzenie Birkhoffa dla specjalnego zbioru liczb pierwszych
Jan Fornal (University of Bristol)
24-01-2019 14:15
, 603
Operator śladu na obszarach Jordana
Krystian Kazaniecki (Uniwersytet Warszawski)
Streszczenie. W latach pięćdziesiątych Gagliardo wykazał, że dla obszaru $\Omega$ z regularnym brzegiem operator śladu z przestrzeni Sobolewa $W^1_1(\Omega)$ do przestrzeni $L^1(\partial \Omega)$ jest surjekcją. Zatem naturalne jest pytanie o istnienie prawego odwrotnego operatora do operatora śladu. Petree udowodnił, że w przypadku półpłaszczyzny $\mathbb{R}x\mathbb{R}_{+}$ nie istnieje prawy odwrotny operator do operatora śladu. Podczas referatu przedstawię prosty dowód twierdzenia Petree, który wykorzystuje tylko pokrycie Whitney'a danego obszaru oraz klasyczne własności przestrzeni Banacha. Następnie zdefiniujemy operator śladu z przestrzeni Sobolewa $W^1_1(K)$, gdzie $K$ jest płatkiem Kocha. Przez pozostałą część mojego referatu skonstruujemy prawy odwrotny do operatora śladu na płatku Kocha. W tym celu scharakteryzujemy przestrzeń śladów jako przestrzeń Arensa-Eelsa z odpowiednią metryką oraz skorzystamy z twierdzenia Ciesielskiego o przestrzeniach funkcji hölderowskich.
15-10-2021 15:30
, https://lu-se.zoom.us/j/65067339175
Entropy Weighted Regularisation: A General Way to Debias Regularisation Penalties
Olof Zetterqvist (University of Gothenburg/Chalmers)
Lasso and ridge regression are well established and successful models for variance reduction and, for the lasso, variable selection. However, they come with a disadvantage of an increased bias in the estimator. In this seminar, I will talk about our general method that learns individual weights for each term in the regularisation penalty (e.g. lasso or ridge) with the goal to reduce the bias. To bound the amount of freedom for the model to choose the weights, a new regularisation term, that imposes a cost for choosing small weights, is introduced. If the form of this term is chosen wisely, the apparent doubling of the number of parameters vanishes, by means of solving for the weights in terms of the parameter estimates. We show that these estimators potentially keep the original estimators’ fundamental properties and experimentally verify that this can indeed reduce bias.
, 604
Operatory Poissona w podwójnej przestrzeni Focka typu B
Patrycja Hęćka (Politechnika Wrocławska)
Przedstawię konstrukcję operatora Poissona typu B działającego w podwójnej przestrzeni Focka typu B. W tym celu przypomnę definicje podwójnych operatorów kreacji i anihilacji oraz wprowadzę pojęcia partycji typu B oraz „rozszerzonych” partycji typu B. Prezentacja stanowi kontynuację odczytu z 27 marca.
27-02-2023 14:15
, HS
Virtual combination of relatively quasiconvex subgroups and separability properties
Ashot Minasyan
Quasiconvex subgroups are basic building blocks of hyperbolic groups, and relatively quasiconvex subgroups play a similar role in relatively hyperbolic groups. If $Q$ and $R$ are relatively quasiconvex subgroups of a relatively hyperbolic group $G$ then the intersection $Q \cap R$ will also be relatively quasiconvex, but the join $\langle Q,R \rangle$ may not be. I will discuss criteria for the existence of finite index subgroups $Q’ \leqslant_f Q$ and $R’ \leqslant_f R$ such that the ``virtual join’’ $\langle Q’, R’ \rangle$ is relatively quasiconvex. This is closely related to separability properties of $G$ and I will present applications to limit groups, Kleinian groups and fundamental groups of graphs of free groups with cyclic edge groups. The talk will be based on joint work with Lawk Mineh.
, 603
Critical patch size and insufficient maximal biomass density level in the Klausmeier vegetation model with non-local dispersal on flat landscapes
Maciej Tadej (Uniwersytet Wrocławski)
We study a reaction–diffusion–dispersal model originally proposed by Klausmeier (1999) to describe the interactions between vegetation biomass and water availability. Motivated by ongoing climate change and the resulting decline in rainfall, our primary focus is on identifying conditions under which stable stationary solutions—corresponding to vegetation survival—exist. From a theoretical standpoint, we successfully construct such solutions and derive precise criteria for their existence. Our analysis also reveals the presence of a critical biomass threshold: below this level, vegetation inevitably goes extinct, with no possibility for recovery. Additionally, we identify a critical patch size—i.e., a minimal domain size—sufficient to sustain vegetation. Domains smaller than this threshold cannot support stable vegetation patterns.
, 604
Sufficient Dimension Reduction in Regression and Classification: An overview and recent results for matrix-valued predictors
Efsthathia Bura (TU Vienna)
, HS
Bohr compactification and type-definable connected component of modules, rings and semidirect product of groups
Mateusz Rzepecki
For a model M and a topological space C we say that a map f: M -> C is definable if for any two disjoint closed sets in C their preimages by f are separable by a definable set. For a definable structure N in a model M we say that f is a definable compactification of N if f is a compactification of N and f is a definable map. We say that a definable compactification of N is universal if every definable compactification of N factors by f via a continuous map. It turns out that if N is a group (Gismatullin, Penazzi & Pillay) or a ring (Gismatullin, Jagiella & Krupiński) then N/N^{00}_M is the universal definable compactification of N, where N^{00}_M is the type-definable connected component of N over M. A module can be represented as N in two ways. The first one is a definable abelian group and a ring that is a part of the language. The second one is a definable abelian group and a definable ring. This talk: In this talk we will prove that for a definable module N (in both senses) N/N^00_M is a universal definable compactification of N (the definition of N^00_M for a module will be given). We will also analyze how N^00_M depends on the type-definable connected component of the abelian group. To do this we will prove a theorem that shows how connected components help in creating sets that are closed under commutative addition (generalization of the proof by Krzysztof Krupiński for approximate rings). Using the theorem, we will describe the type-definable connected component of modules, rings and semidirect products of groups. We will also show that in many cases of structures analyzed during this talk adding homomorphism/monomorphism/automorphism/differentiation to the structure N does not change the type-definable connected component of N. During the talk we will come across a few open questions. This is a joint work with Krzysztof Krupiński and is a part of a bigger project with Grzegorz Jagiella.
, 603
Stratonovich stochastic differential equation with power non-linearity: (non)-uniqueness and selection problem
Georgiy Shevchenko (Kyiv School of Economics)
I will review results regarding a Stratonovich stochastic differential equation $$ X_t=X_0+\int_0^t |X_s|^\alpha\circ d B_s, $$ which was introduced in the physical literature under the name ``heterogeneous diffusion process''. It turns out that equation has properties quite different from its Ito counterpart. Namely, we show that for $\alpha\in(0,1)$ it has infinitely many strong solutions spending zero time at zero. They are given by $X^\theta = \bigl((1-\alpha)B^\theta+(X_0)^{1-\alpha} \bigr)^{1/(1-\alpha)}$, where for $\theta\in(-1,1)$, $B^\theta$ is the $\theta$-skew Brownian motion, and $(x)^{\gamma} = |x|^\gamma \operatorname{sign} x$. It appears that there are no other homogeneous strong Markov solutions to the equation. To address the non-uniqueness, we consider a perturbation of the equation by a small independent noise. It appears that the solution to such equations converge to the solution of initial equation corresponding to $\theta=0$, i.e. the physically symmetric case.
, A.4.1 C-19
Ultrafilters vs measures
Arturo Martinez Celis
In this talk, we will discuss the similarities and differences between ultrafilters and finite additive measures on the natural numbers, with a particular emphasis on the Rudin-Keisler and Rudin-Blass orderings and their generalization to measures.
06-06-2019 12:15
, 606
Testowanie stochastycznego uporządkowania dwóch funkcji przeżycia, II.
Grzegorz Wyłupek
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