Colloquium

  • 2017 Mar 23

    Colloquium: Asaf Shapira (Tel Aviv) - "Removal Lemmas with Polynomial Bounds"

    2:30pm to 3:30pm

    Location: 

    Manchester Building (Hall 2), Hebrew University Jerusalem
    A common theme in many extremal problems in graph theory is the
    relation between local and global properties of graphs. We will
    consider the following variant of this theme: suppose a graph G
    is far (in some well defined sense) from satisfying property P.
    Must G contain a small proof of this fact? We will show that
    for many natural graph properties the answer is Yes. In particular,
    we will show that the answer is Yes whenever P is a semi-algebraic
    graph property, thus conforming a conjecture of Alon.
    Joint work with L. Gishboliner
  • 2017 Mar 16

    Colloquium: Oren Becker (HUJI) Tzafriri Prize Lecture "Equations in permutations and group theoretic local testability"

    2:30pm to 3:30pm

    Location: 

    Manchester Building (Hall 2), Hebrew University Jerusalem
    Abstract: Given two permutations A and B which "almost" commute, are they "close" to permutations A' and B' which really commute? This can be seen as a question about a property the equation XY=YX.
    Studying analogous problems for more general equations (or sets of equations) leads to the notion of "locally testable groups" (aka "stable groups").
  • 2017 Jan 22

    Special colloquium: Laci Babai (Chicago) "Graph isomorphism and coherent configurations: The Split-or-Johnson routine"

    4:00pm to 6:00pm

    Location: 

    Rothberg B220 (CS bldg)
    Coherent configurations" (CCs) are certain highly regular colorings of the directed complete graph. The concept goes back to Schur (1933) who used it to study permutation groups, and has subsequently been rediscovered in other contexts (block designs,
    association schemes, graph canonization).
    CCs are the central concept in the "Split-or-Johnson" (SoJ) procedure, one of the main combinatorial components of the speaker's recent algorithm to test graph isomorphism.
  • 2017 Jan 19

    Special colloquium: Asaf Katz (HUJI Perlman prize) "Sparse equidistribution in unipotent flows"

    4:00pm to 5:00pm

    Location: 

    Manchester building room 2
    Abstract - Equidistribution problems, originating from the classical works of Kronecker, Hardy and Weyl about equidistribution of sequences mod 1, are of major interest in modern number theory.
    We will discuss how some of those problems relate to unipotent flows and present a conjecture by Margulis, Sarnak and Shah regarding an analogue of those results for the case of the horocyclic flow over a Riemann surface. Moreover, we provide evidence towards this conjecture by bounding from above the Hausdorff dimension of the set of points which do not equidistribute.
  • 2016 Dec 29

    Colloquium: Jordan Ellenberg (University of Wisconsin) "The cap set problem"

    2:30pm to 3:30pm

    Location: 

    Manchester Building (Hall 2), Hebrew University Jerusalem
    A very old question in additive number theory is: how large can a subset of Z/NZ be which contains no three-term arithmetic progression? An only slightly younger problem is: how large can a subset of (Z/3Z)^n be which contains no three-term arithmetic progression? The second problem was essentially solved in 2016, by the combined work of a large group of researchers around the world, touched off by a brilliantly simple new idea of Croot, Lev, and Pach.
  • 2016 Dec 22

    Colloquium: Itai Ben Yaakov (Université Claude Bernard - Lyon 1) "Full globally valued fields"

    2:30pm to 3:30pm

    Location: 

    Manchester Building (Hall 2), Hebrew University Jerusalem
    The Globally Valued Fields (GVF) project is a joint effort with E. Hrushovski to understand (standard and) non-standard global fields - namely fields in which a certain abstraction of the product formula holds. One possible motivation is to give a model-theoretic framework
    for various asymptotic distribution results in global fields.
    Formally, a GVF is a field together with a "valuation" in the additive group of an L^1 space, such that the integral of v(a) vanishes for every non-zero a .
  • 2016 Dec 15

    Colloquium: Cy Maor (Toronto) "Asymptotic rigidity of manifolds"

    2:30pm to 3:30pm

    Location: 

    Manchester Building (Hall 2), Hebrew University Jerusalem
    Liouville's rigidity theorem (1850) states that a map $f:\Omega\subset
    R^d \to R^d$ that satisfies $Df \in SO(d)$ is an affine map. Reshetnyak
    (1967) generalized this result and showed that if a sequence $f_n$
    satisfies $Df_n \to SO(d)$ in $L^p$, then $f_n$ converges to an affine
    map.
    In this talk I will discuss generalizations of these theorems to mappings
    between manifolds and sketch the main ideas of the proof (using techniques
    from the calculus of variations and from harmonic analysis).
  • 2016 Dec 08

    Colloquium: Gordon Slade (UBC) "Critical phenomena in statistical mechanics"

    2:30pm to 3:30pm

    Location: 

    Manchester Building (Hall 2), Hebrew University Jerusalem
    The subject of phase transitions and critical phenomena in statistical mechanics is a rich source of interesting and difficult mathematical problems. There has been considerable success in solving such problems for systems in spatial dimension 2, or in high dimensions, but not in dimension 3. This lecture is intended to provide an introduction to recent work that employs a renormalisation group method to study spin systems and self-avoiding walk in dimension 4 (joint with Bauerschmidt and Brydges), as well as long-range versions of these models in dimensions 1,2,3 via an "epsilon expansion."

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