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Monday, February 7, 2022 11:30 am - 11:30 am EST (GMT -05:00)

Algebraic Graph Theory Seminar - Gabriel Coutinho

Title: Optimizing sums of eigenvalues

Speaker: Gabriel Coutinho
Affiliation: Universidade Federal de Minas Gerais
Zoom: Contact Sabrina Lato

Abstract:

In its original proof, Hoffman's well known lower bound to the chromatic number is obtained after replacing several terms of a sum of eigenvalues by the smallest eigenvalue of the graph.

Tuesday, February 8, 2022 3:00 pm - 3:00 pm EST (GMT -05:00)

Graphs and Matroids Seminar - Sophie Spirkl and James Davies

Title: Two counterexamples related to chi-boundedness

Speaker: Sophie Spirkl and James Davies
Affiliation: University of Waterloo
Zoom: Click here or please email Shayla Redlin

Abstract:

This will be a joint talk with two parts.


Sophie Spirkl: I will present a counterexample to the following well-known conjecture: for every k, r, every graph G with clique number at most k and sufficiently large chromatic number contains a triangle-free induced subgraph with chromatic number at least r. 

Thursday, February 10, 2022 10:30 am - 10:30 am EST (GMT -05:00)

Cryptography Reading Group - Siwei Sun

Title: Rotational Differential-Linear Cryptanalysis of ARX Ciphers

Speaker: Siwei Sun
Affiliation: University of Chinese Academy of Sciences
Zoom: Please email Jesse Elliott

Abstract:

The differential-linear attack, combining the power of the two most effective techniques for symmetric-key cryptanalysis, was proposed by Langford and Hellman at CRYPTO 1994. We further extend this framework by replacing the differential part of the attack by rotational differentials.

Thursday, February 10, 2022 1:00 pm - 1:00 pm EST (GMT -05:00)

Algebraic and Enumerative Combinatorics Seminar - Maciej Dolega

Title:  A curious identity between the orthogonal Brezin--Gross--Witten integral and Schur symmetric functions via b-deformed monotone Hurwitz numbers

Speaker: Maciej Dolega
Affiliation: IMPAN
Zoom: Please email Olya Mandelshtam

Abstract:

This talk is intended for an algebraic combinatorial community and no prior knowledge is required. All the difficult words (Hurwitz numbers, KP hierarchy, HCIZ and BGW integrals, Jack symmetric functions, the b-conjecture) will be explained and gently introduced.

Monday, February 14, 2022 11:30 am - 11:30 am EST (GMT -05:00)

Algebraic Graph Theory Seminar - Christino Tamon

Title: On reverse mixing in quantum walks

Speaker: Christino Tamon
Affiliation: Clarkson University
Zoom: Contact Sabrina Lato

Abstract:

A random walk on a connected undirected graph mixes to the principal eigenvector. We consider a continuous-time quantum walk to reverse this process: starting with the principal eigenvector, can we reach an arbitrary vertex if we were to place a loop on the target vertex?

Tuesday, February 15, 2022 4:00 pm - 4:00 pm EST (GMT -05:00)

Graphs and Matroids Seminar - Chun-Hung Liu

Title: Homomorphism counts in robustly sparse graphs

Speaker: Chun-Hung Liu
Affiliation: Texas A&M University
Zoom: Click here or please email Shayla Redlin

Abstract:

For a fixed graph H and for arbitrarily large host graphs G, the number of homomorphisms from H to G and the number of subgraphs isomorphic to H contained in G have been extensively studied when the host graphs are allowed to be dense.

Friday, February 18, 2022 3:30 pm - 3:30 pm EST (GMT -05:00)

Tutte Colloquium - Chandra Chekuri

Title: Densest Subgraph: Supermodularity, Iterative Peeling, and Flow

Speaker: Chandra Chekuri
Affiliation: University of Illinois, Urbana-Champaign
Zoom: Please email Emma Watson

Abstract:

The densest subgraph problem in a graph (DSG), in the simplest form, is the following. Given an undirected graph G = (V, E) find a subset S of vertices that maximizes the ratio |E(S)|/|S| where E(S) is the set of edges with both endpoints in S. DSG and several of its variants are well-studied in theory and practice and have many applications in data mining and network analysis.

Tuesday, March 1, 2022 11:00 am - 11:00 am EST (GMT -05:00)

Graphs and Matroids Seminar - Louis Esperet

Title: Packing and covering balls in planar graphs

Speaker: Louis Esperet
Affiliation: G-SCOP Laboratory
Zoom: Join via http://matroidunion.org/?page_id=2477 or please email Shayla Redlin

Abstract:

The set of all vertices at distance at most r from a vertex v in a graph G is called an r-ball. We prove that the minimum number of vertices hitting all r-balls in a planar graph G is at most a constant (independent of r) times the maximum number of vertex-disjoint r-balls in G.

Thursday, March 3, 2022 1:00 pm - 1:00 pm EST (GMT -05:00)

Algebraic Combinatorics Seminar - Federico Castillo

Title: Lineup polytopes and exclusion principles

Speaker: Federico Castillo
Affiliation: Universidad Catolica de Chile
Zoom link: Contact Logan Crew

Abstract:

The set of all possible spectra of 1-reduced density operators for systems of N particles on a d-dimensional Hilbert space is a polytope called hypersimplex and this is related to Pauli's exclusion principle. If the spectrum of the original density operators is fixed, the set of spectra (ordered decreasingly) of 1-reduced density operators is also a polytope.

Friday, March 4, 2022 3:30 pm - 3:30 pm EST (GMT -05:00)

Tutte Colloquium - Mireille Bousquet-Mélou

Title: Counting planar maps, 50 years after William Tutte

Speaker: Mireille Bousquet-Mélou
Affiliation: CNRS, Université de Bordeaux
Location: MC 5501 or please contact Emma Watson for Zoom link

Abstract:

Every planar map can be properly coloured with four colours. But how many proper colourings has, on average, a planar map with $n$ edges? What if we allow a prescribed number of "monochromatic" edges, the endpoints of which share the same colour? What if we have $q$ colours rather than four?