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Abstract: The Valued Constraint Satisfaction Problem (VCSP) is a well-known combinatorial problem. An instance of VCSP is given by a finite set of variables, a finite domain of labels for the variables, and a sum of functions, each function depending on a subset of the variables. Each function can take finite rational values specifying costs of assignments of labels to its variables or the infinite value, which indicates an infeasible assignment. The goal is to find an assignment of labels to the variables that minimizes the sum. The case when all functions take only values 0 and infinity corresponds to the standard CSP. We study (assuming that $P\neq NP$) how the computational complexity of VCSP depends on the set of functions allowed in the instances, the so-called constraint language. Helped greatly by algebra, massive progress has been made in the last three years on this complexity classification question, and our work gives, in a way, the final answer to it, modulo the complexity of CSPs.

This is joint work with Vladimir Kolmogorov and Michal Rolinek (both from IST Austria).

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Abstract: The classical Analyst's Traveling Salesman Theorem of Peter Jones gives a condition for when a subset of Euclidean space can be contained in a curve of finite length (or in other words, when a "traveling salesman" can visit potentially infinitely many cities in space in a finite time). The length of this curve is given by a sum of quantities called beta-numbers that measure how non-flat the set is at each scale and location. Conversely, given such a curve, the sum of its beta-numbers is controlled by the total length of the curve, giving us quantitative information about how non-flat the curve is. This result and its subsequent variants have had applications to various subjects like harmonic analysis, complex analysis, and harmonic measure. In this talk, we will introduce a version of this theorem that holds for higher dimensional objects other than curves. This is joint work with Raanan Schul.

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Abstract: TBA

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Abstract: (Joint work with A. Le Boudec) The tree almost automorphism groups are non-discrete locally compact completions of the Higman-Thompson groups. The tree almost automorphism groups are independently interesting locally compact groups, and furthermore every group that almost acts on a sufficiently regular rooted tree embeds into one of these groups. We begin by introducing the almost automorphism groups and describing their relationship to the Higman-Thompson groups. We then consider the subgroups such that every element is contained in a compact subgroup; such groups are the topological analogue of torsion subgroups and are called periodic. We show every periodic subgroup is indeed locally elliptic - i.e. every finite set is contained in a compact subgroup. As applications, we recover a result for Thompson's group V as well as a new observation about the Röver group. We finally consider the commensurated subgroups of almost automorphism groups; these subgroups generalize normal subgroups. We show every commensurated closed subgroup of an almost automorphism group is either finite, compact and open, or equal to the entire group. As an application, we obtain new information on the possible lattice envelopes of Thompson's group T.

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Abstract: TBA

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