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Events for the weeks of February 28th – March 12th

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Tue, Mar. 2 10:30am (zoom)	MathClub Al Bronstein (CU-Boulder) X I will use the material from a paper by Shai Simonson and Tara S. Holm. This is part of their abstract: We present a card trick that can be used to review or teach a variety of topics in discrete mathematics. We address many subjects, including permutations, combinations, functions, graphs, depth first search, the pigeonhole principle, greedy algorithms, and concepts from number theory. Using a card trick to teach discrete mathematics
Tue, Mar. 2 1pm (Zoom)	Logic Andrei Bulatov (Simon Fraser University, Canada) X We give a survey on connections between Graph Isomorphism, the CSP, and counting homomorphisms. In the first part we give a brief review of the main approaches to solving the Graph Isomorphism problem and make some observations on how the CSP techniques can be helpful. In the second part we focus on relaxations of graph isomorphisms and how they can be characterized using the numbers of homomorphisms from various graph classes. Isomorphisms, homomorphisms, and some algebra
Tue, Mar. 2 2pm (MATH 350)	Lie Theory Victor Ostrik (University of Oregon) X This talk is based on joint work with M.Khovanov and Y.Kononov. By evaluating a topological field theory in dimension 2 on surfaces of genus 0,1,2 etc we get a sequence. We investigate which sequences occur in this way depending on the assumptions on the target category. Two dimensional topological field theories and partial fractions
Wed, Mar. 3 10am (via zoom)	Math Phys Bernardo Uribe (Universidad del Norte in Barranquilla, Colombia) X In this talk I will present how topological and representation theoretical constraints provide relevant information in the study of electronic properties of crystals. I will give applications using materials with non-symmorphic hexagonal symmetries. The contents of the talk can be read on: arXiv:2005.02959 and arXiv:2102.09970. Meeting ID: 940 0255 3301 Passcode: 790356 Topological electronic structure and Weyl points in nonsymmorphic hexagonal materials
Thu, Mar. 4 9am (Zoom (vir…	Rep Theory Simon Wood (Cardiff University) X Vertex operator algebras exhibit a feature much like Lie algebras in that they admit too many modules for the category of all their modules to exhibit nice structure. However, good choices of module category can lead to categories with very rich structure. For example the categories of admissible modules over rational vertex operator algebras are modular tensor categories, as proved by Huang. I will present some recent work on making the study of vertex operator algebra module categories more tractable by replacing them by Hopf algebras, an arguably simpler algebraic structure. The guiding example will be the free boson. There is always more that can be learnt from the free boson
Thu, Mar. 4 9:10am (Zoom 9…	Ulam Alexandr Kazda X Valued CSP is an optimization version of the Constraint Satisfaction Problem. Instead of finding any homomorphism from A to B, our goal in VCSP is to find a homomorphism that has a low cost. The complexity of VCSP is well understood with a dichotomy proved by Kolmogorov, Krokhin and Rolínek in 2015. In this talk we will look at how to define the category of valued structures to get nice products in this cateogory. In particular, we will want weighted polymorphisms of a structure A to correspond to mappings from powers of A to A. This will require generalizing the notion of valued relational structure. The correct category for valued CSP
Tue, Mar. 9 4:30pm (MATH 3…	Lie Theory Liron Speyer (Okinawa Institute of Science and Technology) X I will first give a brief survey of some previous results with Louise Sutton, in which we found a large family of decomposable Specht modules for the Hecke algebra of type $B$ indexed by `bihooks'. We conjectured that outside of some degenerate cases, our family gave all decomposable Specht modules indexed by bihooks. There, our methods largely relied on some hands-on computation with Specht modules, working in the framework of cyclotomic KLR algebras. I will then move on to discussing a recent project with Rob Muth and Louise Sutton, in which we have studied the structure of these Specht modules. By transporting the problem to one for Schur algebras via a Morita equivalence of Kleshchev and Muth, we are able to give all composition factors (including their grading shifts), and show that in most characteristics, these Specht modules are in fact semisimple. In some other small characteristics, we can explicitly determine their structures, including some in which the modules are `almost semisimple'. I will present this story, with some running examples that will help the audience keep track of what's going on. Semisimple Specht modules indexed by bihooks (Unusual time: 4:30pm)
Thu, Mar. 11 9:10am (Zoom 9…	Ulam Alexandr Kazda X It is known that minion homomorphisms give very natural complexity reductions between (P)CSPs (Barto, Bulín, Opršal, Krokhin, 2019). We will explore how to translate these techniques into the valued CSP situation. Convex geometry will make an appearance. Minion homomorphisms and valued CSP