Contact Info
Combinatorics & Optimization
University of Waterloo
Waterloo, Ontario
Canada N2L 3G1
Phone: 5198884567, ext 33038
PDF files require Adobe Acrobat Reader.
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Title: Excludedminor characteristics
Speaker: Jim Geelen Affiliation: University of Waterloo Location: MC 5501Abstract: We review the excludedminor characterizations for matroids over small fields and discuss the obstacles towards finding such a characterization for GF(5)representability.
Title: On the complexity of quantum partition functions
Speaker: David Gosset Affiliation: University of Waterloo Location: MC 5501Abstract: Quantum complexity theory has been intertwined with the study of quantum manybody systems ever since Kitaev's insight that computing their ground energies is an intractable quantum constraint satisfaction problem that is complete for a quantum generalization of NP.
Title: Approximation algorithms for dense quantum Hamiltonians using convex relaxations
Speaker: Anirban Chowdhury Affiliation: University of Waterloo Location: MC 5479Abstract: Computing groundstate energy and partition functions for quantum Hamiltonian systems are problems of broad applicability in physics. These are also natural generalizations of wellstudied classical computational tasks such as maximum constraint satisfaction and computing partition functions of Ising models. In this talk, I will present new classical approximation algorithms for these problems in the case of dense quantum Hamiltonians.
Title: Quantum walk state transfer on a hypercube
Speaker: Martin Stefanak Affiliation: Czech Technical University Location: Please contact Sabrina Lato for Zoom linkAbstract: We investigate state transfer on a hypercube by means of a quantum walk where the sender and the receiver vertices are marked by weighted loops.
Title: Extended Formulations for the Colonel Blotto Game
Speaker: Ian DeHaan Affiliation: University of Waterloo Location: MC 6029Abstract: Suppose you want to replace Doug Ford as premier. To beat him, you need to win as many seats as possible. You have some finite amount of resources to allocate across all seats  if you spend more on a seat than Ford does, you will win that seat. How should you allocate your resources to maximize your expected number of seats?
Title: Online UnrelatedMachine Load Balancing and Generalized Flow with Recourse
Speaker: Shi Li Affiliation: University at Buffalo Location: MC 5501Abstract: I will present the online algorithms for unrelatedmachine load balancing problem with recourse. First, we shall present a (2+\epsilon)competitive algorithm for the problem with O_\epsilon(\log n) amortized recourse per job.
Title: Lsystems and the Lovasz number
Speaker: William Linz Affiliation: University of South Carolina Location: Please contact Sabrina Lato for Zoom linkAbstract: For positive integers n and k, an Lsystem is a collection of kuniform subsets of a set of size n whose pairwise intersection sizes all lie in in the set L. The maximum size of an Lsystem is equal to the independence number of a certain union of graphs in the Johnson scheme. The Lovasz number is a semidefinite programming approximation of the independence number of a graph. In this talk, we survey the relationship between the maximum size of an Lsystem and the Lovasz number, illustrating examples both where the Lovasz number is a good approximation and where it is a bad approximation.
Title: Research in Applications
Speaker: Ricardo Fukasawa Affiliation: University of Waterloo Location: MC 5479Abstract: In this talk I will present my personal experiences in doing research involving applications. I will go over some of my work, presenting some of the key aspects that are involved, and trying to take stock of a few lessons learned.
Title: Monomial ideals, Galois closures, and Hilbert schemes of points
Speaker: Matthew Satriano Affiliation: University of Waterloo Location: MC 5501 and Zoom  please contact Oliver Pechenik for the Zoom linkAbstract: Manjul Bhargava and the speaker introduced a functorial Galois closure operation for finiterank ring extensions, generalizing constructions of Grothendieck and KatzMazur. In this talk, we use Galois closures to construct new components of Hilbert schemes of points, which are fundamental objects in algebraic geometry whose component structure is largely mysterious. We answer a 35 year old open problem posed by Iarrobino by constructing an infinite family of low dimensional components. This talk is based on joint work with Andrew Staal. No prior knowledge of Hilbert schemes will be assumed.
Title: To be announced
Speaker: Rutger Campbell Affiliation: University of Waterloo Location: MC 5501Title: Error bounds for conic feasibility problems: case studies on the exponential cone
Speaker: Ting Kei Pong Affiliation: The Hong Kong Polytechnic University Location: MC 5501Abstract: Conic feasibility problems naturally arise from linear conic programming problems. An understanding of error bounds for these problems is instrumental in the design of termination criteria for conic solvers and the study of convergence rate of algorithms.
Title: Partial geometric designs, directed strongly regular graphs, and association scheme
Speaker: Sung Song Affiliation: Iowa State University Location: Please contact Sabrina Lato for Zoom linkAbstract: A partial geometric design with parameters $(v, b, k, r; \alpha, \beta)$ is a tactical configuration $(P, \mathcal{B})$ (with $P=v$, $\mathcal{B}=b$, every point $p\in P$ belonging to $r$ blocks, and every block $B\in\mathcal{B}$ consisting of $k$ points) satisfying the property:
{for any pair $(p, B)\in P\times \mathcal{B}$, the number of flags $(q, C)$ with $q\in B$ and $C\ni p$ equals to $\alpha \mbox{ if } p\notin B$ and to $\beta \mbox{ if } p\in B$.}
Neumaier studied partial geometric designs in detail in his article, ``$t\frac12$designs," [JCT A {\bf 28}, 226248 (1980)]. He investigated their connection with stronglyregular graphs and gave various characterizations of partial geometries, bipartite graphs, symmetric 2designs, and transversal designs in terms of partial geometric designs.
Title: A PrimalDual Extension of the GoemansWilliamson Algorithm for the Weighted Fractional Cut Covering Problem
Speaker: Nathan Benedetto Proenca Affiliation: University of Waterloo Location: MC 6029Abstract:
A cut in a graph \(G = (V, E)\) is a set of edges which has precisely one endpoint in \(S\), for a given subset \(S\) of \(V\). The fractional cutcovering number is the optimal value of a linear programming relaxation for the problem of covering each edge by a set of cuts. We define a semidefinite programming relaxation of fractional cut covering whose approximate optimal solutions may be rounded into a fractional cut cover via a randomized algorithm.
Title: An invitation to monotone operators and their applications in optimization
Speaker: Walaa Moursi Affiliation: University of Waterloo Location: MC 5479Abstract: In this talk, I give an overview of the theory of monotone operators and its connection to optimization algorithms. This talk is a good introduction to how abstract theoretical results serve as bases for successful algorithms in practice.
Title: Poset subHopf algebras from growth models in causal set theory and quantum field theory
Speaker: Karen Yeats Affiliation: University of Waterloo Location: MC 5501 and Zoom  please contact Oliver Pechenik for the Zoom linkAbstract: In a story some of you have heard from me before, we get subHopf algebras of the ConnesKreimer Hopf algebra of rooted trees from certain simple tree classes which correspond to solutions to combinatorial analogues of DysonSchwinger equations in quantum field theory. Another important subHopf algebra of the ConnesKreimer Hopf algebra is the ConnesMoscovici Hopf algebra which can be viewed as coming from rooted trees grown by adding leaves.
Title: The rank of sparse symmetric matrices over arbitrary fields
Speaker: Noela Müller Affiliation: TU/e Eindhoven University of Technology Location: MC 5501Abstract: Consider a sequence of sparse ErdösRényi random graphs (G_{n,d/n})_n on n vertices with edge probability d/n. Moreover, we equip the edges of G_{n,d/n} with prescribed nonzero edge weights chosen from an arbitrary field F.
Title: Diagonal coefficients of Kirchhoff polynomials of 2kregular graphs and the proof of the c_2 completion conjecture
Speaker: Karen Yeats Affiliation: University of Waterloo and Perimeter Institute Location: Please contact Sabrina Lato for Zoom linkAbstract: I have for many years been interested in graph invariants with the same symmetries as the Feynman period. Recently Erik Panzer found a new such invariant coming from a particular coefficient of the Martin polynomial. Together we used this to prove an over 10 year old conjecture on an arithmetic graph invariant known as the c_2 invariant, and came to understand that diagonal coefficients of Kirchhoff polynomials tie together many of the known graph invariants with the symmetries of Feynman periods and unlock previously inaccessible proofs.
Joint work with Erik Panzer.
Title: A PrimalDual Extension of the GoemansWilliamson Algorithm for the Weighted Fractional Cut Covering Problem, Part II
Speaker: Nathan Benedetto Proenca Affiliation: University of Waterloo Location: MC 6029Abstract: A cut in a graph \(G = (V, E)\) is a set of edges which has precisely one endpoint in \(S\), for a given subset \(S\) of \(V\). The fractional cutcovering number is the optimal value of a linear programming relaxation for the problem of covering each edge by a set of cuts. We define a semidefinite programming relaxation of fractional cut covering whose approximate optimal solutions may be rounded into a fractional cut cover via a randomized algorithm.
Title: Restricted Intersections and the Sunflower Problem
Speaker: Jeremy Chizewer Affiliation: University of Waterloo Location: MC 5479Abstract: A sunflower with $r$ petals is a collection of $r$ sets over a ground set $X$ such that every element in $X$ is in no set, every set, or exactly one set. Erdos and Rado showed that a family of sets of size $n$ contains a sunflower if there are more than $n!(r1)^n$ sets in the family. Alweiss et al. and subsequently Rao improved this bound to $(O(r \log(rn))^n$.
Title: Minors of random representable matroid over finite fields
Speaker: Jane Gao Affiliation: University of Waterloo Location: MC 5479Abstract: Consider a random n by m matrix A over GF(q) where every column has k nonzero elements, and let M[A] be the matroid represented by A. In the case that q=2, Cooper, Frieze and Pegden (RSA 2019) proved that given a fixed binary matroid N, if k is sufficiently large, and m/n is sufficiently large (both depending on N), then whp. M[A] contains N as a minor. We improve their result by determining the sharp threshold (of m/n) for the appearance of a fixed qnary matroid N as a minor of M[A], for every k\ge 3, and every prime q. This is joint work with Peter Nelson.
Title: Ramsey degrees, big and small
Speaker: Andy Zucker Affiliation: University of Waterloo Location: MC 5501Abstract: Many of the seminal results in finite Ramsey theory can be phrased by saying that a certain class of finite structures has the Ramsey property, such as the ordinary finite Ramsey theorem (the class of finite linear orders), the dual Ramsey theorem (the class of finite lexordered Boolean algebras), the GrahamLeebRothschild theorem (the class of lexordered, finitedimensional vector spaces over a fixed finite field), and the NesetrilRodl theorem (the class of finite ordered trianglefree graphs, among many others).
Combinatorics & Optimization
University of Waterloo
Waterloo, Ontario
Canada N2L 3G1
Phone: 5198884567, ext 33038
PDF files require Adobe Acrobat Reader.
The University of Waterloo acknowledges that much of our work takes place on the traditional territory of the Neutral, Anishinaabeg and Haudenosaunee peoples. Our main campus is situated on the Haldimand Tract, the land granted to the Six Nations that includes six miles on each side of the Grand River. Our active work toward reconciliation takes place across our campuses through research, learning, teaching, and community building, and is coordinated within our Office of Indigenous Relations.