Events

Title: Combinatorial Nullstellensatz and the Erdős box problem

Abstract: In the talk, I will show how Lasoń’s generalization of Alon’s Combinatorial Nullstellensatz can be used to obtain lower bounds on Turán numbers of complete r-partite r-uniform hypergraphs. As an example, I will give a short and simple explicit construction of a hypergraph free of copies of the complete r-partite r-uniform  hypergraph with parts of size 2, thereby providing a lower bound for the so-called Erdős box problem. This asymptotically matches best known bounds when r ≤ 4. 

Title:Coefficient Positivity and Analytic Combinatorics

 Abstract: Given a rational function analytic at the origin, are its power series coefficients positive? We first motivate this question with historical background, examples and connections to decidability questions in combinatorics. We then present a method for proving positivity for a certain class of multivariate rational functions, by constructing explicit error bounds for an asymptotic expansion using analytic combinatorics in several variables. After applying our method to a couple of examples in the literature, including re-proving a (recently proved) conjecture of Gillis, Reznick and Zeilberger from 1983, we discuss the feasibility of completing certain steps in our analysis using methods from computer algebra.

There will be a pre-seminar presenting relevant background at the beginning graduate level starting at 1pm

Abstract: In this talk, we will cover the paper of Svensson and Tarnawski that shows that perfect matching in general (non-bipartite) graphs in is quasi-NC. Similar to the work of Fenner, Gurjar and Thierauf (covered earlier in the reading group), the approach is to derandomize the isolation lemma for the perfect matching polytope by applying weight functions to iteratively restrict to subfaces of the polytope. However, the perfect matching polytope in general graphs is not as well-structured as it is in bipartite graphs. The faces of the polytope no longer correspond to subgraphs and now involve additional tight odd set constraints that need to be dealt with. This makes it so that a cycle with non-zero circulation may still exist in the support of the new face. Additionally, the existence of odd cycles in the graph breaks the cycle counting argument used in the paper of Fenner, Gurjar, Thierauf. We will see how Svensson and Tarnawski deal with these issues in the talk.

Title: Purifying arbitrarily noisy quantum states

Abstract: Quantum state purification is the task of recovering a nearly pure copy of an unknown pure quantum state using multiple noisy copies of the state. We derive an efficient purification procedure based on the swap test for qudits of any dimension, starting with any initial error parameter. For constant initial error parameter and dimension, our procedure has sample complexity asymptotically optimal in the final error parameter, and almost matches the known optimal protocol for qubits. Our protocol has a simple recursive structure that can be applied when the states are provided one at a time in a streaming fashion, requiring only a small quantum memory to implement.  Joint work with Andrew Childs, Honghao Fu, Zhi Li, Maris Ozols, Vedang Vyas. 

Title: Constructing cospectral graphs - a ``Prehistory’'

Abstract: I will present some old constructions of cospectral graphs, due Brendan McKay and myself. I will also describe how we found some of these.

Title: Unavoidable flats in matroids representable over a finite field

Abstract: For a positive integer k and finite field F, we prove that every simple F-representable matroid with sufficiently high rank has a rank-k flat which either is independent, or is a projective or affine geometry over a subfield of F.  As a corollary, we obtain the following Ramsey theorem: given an F-representable matroid of sufficiently high rank and any 2-colouring of its points,  there is a monochromatic rank-k flat.  This is joint work with Jim Geelen and Peter Nelson. 

Title: Cyclotomic generating functions

Abstract: It is a remarkable fact that for many statistics on finite sets of combinatorial objects, the roots of the corresponding generating function are each either a complex root of unity or zero. These and related polynomials have been studied for many years by a variety of authors from the fields of combinatorics, representation theory, probability, number theory, and commutative algebra. We call such polynomials *cyclotomic generating functions* (CGFs). We will review some of the many known examples and give results to classify the asymptotic behavior of their coefficient sequences in certain regimes.

Joint work with Sara Billey.

There will be a pre-seminar presenting relevant background at the beginning graduate level starting at 1pm.

Abstract: In this talk, we will cover the paper of Svensson and Tarnawski that shows that perfect matching in general (non-bipartite) graphs in is quasi-NC. Similar to the work of Fenner, Gurjar and Thierauf (covered earlier in the reading group), the approach is to derandomize the isolation lemma for the perfect matching polytope by applying weight functions to iteratively restrict to subfaces of the polytope. However, the perfect matching polytope in general graphs is not as well-structured as it is in bipartite graphs. The faces of the polytope no longer correspond to subgraphs and now involve additional tight odd set constraints that need to be dealt with. This makes it so that a cycle with non-zero circulation may still exist in the support of the new face. Additionally, the existence of odd cycles in the graph breaks the cycle counting argument used in the paper of Fenner, Gurjar, Thierauf. We will see how Svensson and Tarnawski deal with these issues in the talk.

Title: Rotation-invariant web bases from hourglass plabic graphs and symmetrized six-vertex configurations

Abstract: Many combinatorial objects with strikingly good enumerative formulae also have remarkable dynamical behavior and underlying algebraic structure. In this talk, we consider the promotion action on certain rectangular tableaux and explain its small, predictable order by reinterpreting promotion as a rotation in disguise. We show how the search for this visual explanation of combinatorial dynamics led to the solution of an algebraic problem involving web bases and a generalization of the six-vertex model of statistical physics. We find that this framework includes several unexpected combinatorial objects of interest: alternating sign matrices, plane partitions, and their symmetry classes (joint with Ashleigh Adams). This talk is based primarily on joint work with Christian Gaetz, Stephan Pfannerer, Oliver Pechenik, and Joshua P. Swanson.

Title: Hypercube decompositions and combinatorial invariance for Kazhdan-Lusztig polynomials

Abstract: Kazhdan-Lusztig polynomials are of foundational importance in geometric representation theory. Yet the Combinatorial Invariance Conjecture, due to Lusztig and to Dyer, suggests that they only depend on the combinatorics of Bruhat order. I'll describe joint work with Grant Barkley in which we adapt the hypercube decompositions introduced by Blundell-Buesing-Davies-Veličković-Williamson to prove this conjecture for Kazhdan-Lusztig R-polynomials in the case of elementary intervals in the symmetric group. This significantly generalizes the main previously known case of the conjecture, that of lower intervals.

There will be a pre-seminar presenting relevant background at the beginning graduate level starting at 1pm.