Hermann Þórisson (30/11/2018)

Valentina Giangreco, nóvember 3, 2018

Málstofa í stærðfræði

Fyrirlesari: Hermann Þórisson, University of Iceland

Titill: What is typical?

Staðsetning: Naustið (Endurmenntun)
Tími: Föstudagur 30. nóvember kl. 11.40


The word “typical” is often used in a loose sense for events in a stationary random process (such as occurrences of heads in repeated coin tosses). This concept can be made precise using so-called Palm version of the process. It is however not well known that there are in fact two Palm versions and that it is the less known version that captures the typicality property. So using the well known version is flawed except in the special case when the two versions coincide.


In this talk the elementary example of repeated coin tosses (indexed by the integers) will be used to make transparent what the issue is.


In the latter half of the talk we shift drastically to random measures on a rather general class of Abelian groups (in the coin tossing example the  group is the integers under addition and the random measure is formed by mass points of size one at the heads). After giving the formal definition of the two Palm versions we present a theorem motivating the claim that it is the less known Palm version that captures the typicality property. If time allows we skim through the proof which relies on the concepts of shift-coupling and mass-stationarity.

Watse Sybesma (23/11/2018)

Valentina Giangreco, október 29, 2018

Málstofa í stærðfræði

Fyrirlesari: Watse Sybesma, University of Iceland

Titill:  Black holes and good vibrations

Staðsetning: Naustið (Endurmenntun)
Tími: Föstudagur 23. Nóvember kl. 11.40


Matter falling into a black hole is a dynamical process that can be described by a complicated wave equation, which has to be disentangled from a system of PDEs. Computing the eigenvalues of such a wave equation allows one to obtain the characteristic time it takes for this process to take place, which physically is an interesting quantity. However, in general it is very hard to solve the wave equation or even to disentangle the initial system of PDEs. In this talk I will introduce a series of ways one can approximate and solve these types of problems.

Aruna Rajagopal (22/11/2018)

Valentina Giangreco, október 28, 2018

Math Phys seminar

Fyrirlesari: Aruna Rajagopal, University of Iceland

Titill: Shock and Rarefaction Waves in Out of Equilibrium Lifshitz Fluids

Staðsetning: VR-II 155
Tími: Fimmtudaginn 22. nóvember 22 kl. 13.30


Motivated by the recent developments and interest in the emergence of a non equilibrium steady state or NESS, for a relativistic fluid, and its possible holographic dual, we follow a similar framework to study the properties of such a state for a non-relativistic Lifshitz fluid with a general scaling exponent. This is carried out by solving the Riemann problem for the NESS which develops between two semi-infinite heat reservoirs that are brought into contact, and studying the shock and rarefaction waves which emerge as solutions to this problem.

Hjalti Þór Ísleifsson (16/11/2018)

Valentina Giangreco, október 25, 2018

Málstofa í stærðfræði

Fyrirlesari: Hjalti Þór Ísleifsson, University of Iceland

Titill: Veik grannmynstur á Banach rúmum.

Staðsetning: Naustið (Endurmenntun)
Tími: Föstudagur 16. Nóvember kl. 11.40


Við byrjum á að skilgreina veika grannmynstrið á normrúmum og veika* grann- mynstrið á tvírúmum þeirra. Grundavallar eiginleikar þessarra grannmynstra verða kynntir til en sérstök áhersla verður á að skoða þjappleika. Við sönnum setningu Banachs og Alaoglu um að lokaða einingarkúla tvírúms sé þjöppuð í veika* grannmynstrinu. Svo fjöllum við um sjálfhverf Banach rúm, kynnumst grundvallareiginleikum þeirra og skoðum setningu Milman og Pettis sem gefur nægjanlegt rúmfræðilegt skilyrði fyrir því að Banach rúm sé sjálfhverft. Við sönnum setningu Kakutani um að einingarkúla Banach rúms sé þjöppuð í veika grannmynstrinu þá og því aðeins að rúmið sé sjálfhverft. Loks skoðum við setningu Eberlein og Smulian um að þrjár gerðir þjappleika séu jafngildar í normrúmum með veika grannmynstrinu.


Þórður Jónsson (02/11/2018)

Valentina Giangreco, október 24, 2018

Málstofa í stærðfræði

Fyrirlesari: Þórður Jónsson, University of Iceland

Titill: The structure of the spatial slices of 3-dimensional causal triangulations

Staðsetning: Naustið (Endurmenntun)
Tími: Föstudagur 2. Nóvember kl. 11.40


We show that there is a bijection between the spatial slices of 3-dimensional causal triangulations and a class of two-dimensional cell complexes satisfying some simple conditions.  The talk will be preceded by a short introduction to the subject.

Daniel Areán (24/10/2018)

Valentina Giangreco, október 22, 2018

Math Phys seminar

Fyrirlesari: Daniel Areán, IFT Madrid

Titill: Breaking translations in AdS/CMT: Holographic Bad Metals

Staðsetning: Naustið (Endurmenntun)
Tími: Wednesday October 24, 13.30 – 14.30


We present holographic models realizing the pseudo-spontaneous breaking of translations. We study the electrical transport of these solutions finding that they reproduce features characteristic of the bad metallic transport observed in the cuprates.

Eggert Briem (19/10/2018)

Valentina Giangreco, október 12, 2018

Málstofa í stærðfræði

Fyrirlesari: Eggert Briem, University of Iceland

Titill: Gelfand Theory for Real Banach Algebras

Staðsetning: Naustið (Endurmenntun)
Tími: Föstudagur 19. Óktober kl. 11.40


A real Banach algebra is a Banach algebra over the reals. We will only consider commutative Banach algebras with unit. An example is the algebra of continuous functions, f , on the unit disc, analytic in the interior of the disc, satisfying f (z) = f (z). The norm on the algebra is the sup-norm.

Another example is the algebra of continuously differentiable real-valued functions on the unit interval with the norm given by

∥f∥ = ∥f∥∞ +∥f′∥∞

According to Gelfand theory, a commutative Banach algebra A with unit, over the complex numbers, can be represented as an algebra of continuous complex valued functions on a compact Hausdorff space X, with

sup_{x ∈ X}|ã(x)| = r(a) := lim∥a^n∥^1/n

for a ∈ A. Here X is the space of multiplicative linear functionals on A, equipped with the w∗-topology, and ã(x) = x(a) for x ∈ X.

This result also holds for real Banach algebras. Furthermore, the representati- on consists of real valued functions if and only if

r(a^2) ≤ r(a^2 +b^2)      a,b ∈ A.
We will prove this using only real Banach space theory. If there is time we

will also talk about the general case where there is no condition on A.


Thomas Selig (12/10/2018)

Valentina Giangreco, október 2, 2018

Málstofa í stærðfræði

Fyrirlesari: Thomas Selig, University of Iceland

Titill: The Abelian sandpile model on permutation graphs

Staðsetning: Naustið (Endurmenntun)
Tími: Föstudagur 12. Óktober kl. 11.40


A permutation graph is a graph whose edges are given by the inversions of a permutation. The Abelian sandpile model (ASM) is a Markov chain on the set of so-called configurations of a graph. Of particular interest are the recurrent configurations, i.e. those that appear infinitely often in the long-time running of the model. We exhibit a bijection between the set of recurrent configurations for the ASM on permutation graphs and the set of tiered trees, introduced by Duggan et al. This provides a new bijective proof of a famous result linking the level polynomial of the ASM to the ubiquitous Tutte polynomial. We also show a link between the minimal recurrent configurations and the set of complete non-ambiguous binary trees, introduced by Aval et al.

Tobias Zingg (04/10/2018)

Valentina Giangreco, október 2, 2018

Math Phys seminar

Fyrirlesari: Tobias Zingg, Nordita

Titill: plasmons and physical density-density response from holography

Staðsetning: Aðalbygging A-069

Tími: Thursday October 4, at 1.20 -2.20 pm


The holographic correspondence can be extended to identify plasmonic excitations in strongly correlated systems. These modes have relevance in the growing field of plasmonics, and play also a crucial role in relating properties of holographic ‘screened’ density-density response response functions to ‘physical’ density-density response functions, with the latter being the quantity actually related to experimental observation.

Þorsteinn Jónsson (02/07/18)

Anders Claesson, júní 29, 2018

Málstofa í stærðfræði

Fyrirlesari: Þorsteinn Jónsson, University of Guelph

Titill: Hnikaaðferðir til að læra dreififöll gagna

Staðsetning: V-147 (VR-II)
Tími: Mánudagur 2. júlí kl. 10:30


Á þessari málstofu mun ég kynna safn aðferða sem að leyfir okkur að skilgreina tölfræðileg líkön sem lýsa líkindadreifingum sem búa til gögn.
Til þessa kynnum við tauganet sem hraða, en ótrúlega áhrifaríka leið til að leysa hnikaverkefni fyrir vel valið kostnaðarfall.
Ég mun ræða tvær mismunandi aðferðir til þess að skilgreina þetta kostnaðarfall ásamt því að sýna áhugaverðar niðurstöður.