• 2018 May 23

    Analysis Seminar: Ori Gurel-Gurevich (HUJI) "Random walks on planar graphs"

    12:00pm to 1:00pm


    Ross Building, Room 70
    Title: Random walks on planar graphs Abstract: We will discuss several results relating the behavior of a random walk on a planar graph and the geometric properties of a nice embedding of the graph in the plane (e.g. a circle packing of the graph). An example of such a result is that for a bounded degree graph, the simple random walk is recurrent if and only if the boundary of the nice embedding is a polar set (that is, Brownian motion misses it almost surely). No prior knowledge about random walks, circle packings or Brownian motion is required.
  • 2018 May 22

    Barak Weiss (TAU): New examples for the horocycle flow on the moduli space of translation surfaces

    2:15pm to 3:15pm

    A longstanding open question concerning the horocycle flow on moduli space of translation surfaces, is whether one can classify the invariant measures and orbit-closures for this action. Related far-reaching results of Eskin, Mirzakhani and Mohammadi indicated that the answer might be positive. However, in recent work with Jon Chaika and John Smillie, we find unexpected examples of orbit-closures; e.g. orbit closures which are not generic for any measure, and others which have fractional Hausdorff dimension. Such examples exist even in genus 2.
  • 2018 May 22

    T&G: Elisheva Adina Gamse (Toronto), The moduli space of parabolic vector bundles over a Riemann surface

    12:00pm to 1:30pm


    Room 110, Manchester Buildling, Jerusalem, Israel
    Let $\Sigma$ be a Riemann surface of genus $g \geq 2$, and p be a point on $\Sigma$. We define a space $S_g(t)$ consisting of certain irreducible representations of the fundamental group of $\Sigma \setminus p$, modulo conjugation by SU(n). This space has interpretations in algebraic geometry, gauge theory and topological quantum field theory; in particular if Σ has a Kahler structure then $S_g(t)$ is the moduli space of parabolic vector bundles of rank n over Σ.
  • 2018 May 21

    Combinatorics: Daniel Kalmanovich and Or Raz (HU) "2 talks back-to-back"

    11:00am to 12:30pm


    IIAS, Eilat hall, Feldman Building, Givat Ram
    First speaker: Daniel kalmanovich, HU Title: On the face numbers of cubical polytopes Abstract: Understanding the possible face numbers of polytopes, and of subfamilies of interest, is a fundamental question. The celebrated g-theorem, conjectured by McMullen in 1971 and proved by Stanley (necessity) and by Billera and Lee (sufficiency) in 1980-81, characterizes the f-vectors of simplicial polytopes.
  • 2018 May 21

    HD-Combinatorics Special Day on "Stability in permutations" (organized by Oren Becker)

    (All day)


    Room 130, IIAS, Feldman Building, Givat Ram

    Both talks will be given by Oren Becker.
    9:00 - 10:50
    Title: Proving stability via hyperfiniteness, graph limits and invariant random subgroups

    Abstract: We will discuss stability in permutations, mostly in the context of amenable groups. We will characterize stable groups among amenable groups in terms of their invariant random subgroups. Then, we will introduce graph limits and hyperfinite graphings (and some theorems about them), and show how the aforementioned characterization of stability follows.

    14:00 - 16:00
  • 2018 May 17

    Basic Notions - Benjamin Weiss: "All ergodic systems have the Weak Pinsker property" Part 2

    4:00pm to 5:30pm


    Ross 70
    Second part of the talk from last week: An ergodic system (X;B; μ; T) is said to have the weak Pinsker property if for any ε > 0 one can express the system as the direct product of two systems with the first having entropy less than ε and the second one being isomorphic to a Bernoulli system. The problem as to whether or not this property holds for all systems was open for more than forty years and has been recently settled in the affirmative in a remarkable work by Tim Austin. I will begin by describing why Jean-Paul formulated this prob-