Friday, February 18, 2022 — 3:30 PM EST

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, February 15, 2022 — 4:00 PM EST

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.

Monday, February 14, 2022 — 11:30 AM EST

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?

Thursday, February 10, 2022 — 1:00 PM EST

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.

Thursday, February 10, 2022 — 10:30 AM EST

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.

Tuesday, February 8, 2022 — 3:00 PM EST

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. 

Monday, February 7, 2022 — 11:30 AM EST

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.

Friday, February 4, 2022 — 3:30 PM EST

Title: The Problem Landscape of SIDH

Speaker: David Jao
Affiliation: University of Waterloo
Zoom: Please email Emma Watson

Abstract:

The Supersingular Isogeny Diffie-Hellman protocol (SIDH) has recently been the subject of increased attention in the cryptography community. Conjecturally quantum-resistant, SIDH has the feature that it shares the same data flow as ordinary Diffie-Hellman: two parties exchange a pair of public keys, each generated from a private key, and combine them to form a shared secret.

Thursday, February 3, 2022 — 1:00 PM EST

Title: Semidistrim Lattices

Speaker: Colin Defant
Affiliation: Princeton University
Zoom: Please email Olya Mandelshtam

Abstract:

This talk will introduce semidistrim lattices, which generalize semidistributive lattices and trim lattices; these two families, in turn, generalize distributive lattices. We will discuss structural, topological, and dynamical properties of semidistrim lattices. In particular, we will see how one can define a certain bijective operator on a semidistrim lattice called rowmotion; this definition unifies the definition that Barnard gave for semidistributive lattices and the definition that Thomas and Williams gave for trim lattices.

Tuesday, February 1, 2022 — 3:00 PM EST

Title: Strong components of the directed configuration model

Speaker: Matthew Coulson
Affiliation: University of Waterloo
Zoom: http://matroidunion.org/?page_id=2477 or please email Shayla Redlin

Abstract:

We study the behaviour of the largest components of the directed configuration model in the barely subcritical regime. We show that with high probability all strongly connected components in this regime are either cycles or isolated vertices and give an asymptotic distribution of the size of the kth largest cycle. This gives a configuration model analogue of a result of Luczak and Seierstad for the binomial random digraph.

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