# Events by date

## Friday, September 16, 2022

### Combinatorial Optimization Reading Group - David Kalichman

Friday, September 16, 2022 — 1:00 PM EDT

Title: Stochastic Probing with Applications

Speaker: David Kalichman Affiliation: University of Waterloo Location: MC 6029 or contact Rian Neogi for Zoom link

Abstract:  We will explore a stochastic probing problem. Given a set of elements which have weights and independent probabilities of being "active," the goal is to construct a subset of active elements of maximum weight. To form such a set, we must "probe" elements sequentially to determine whether they are active.

### Tutte Colloquium - Lap Chi Lau

Friday, September 16, 2022 — 3:30 PM EDT

Title: Cheeger Inequalities for Vertex Expansion and Reweighted Eigenvalues

Speaker: Lap Chi Lau Affiliation: University of Waterloo Location: MC 5501

Abstract:

The classical Cheeger's inequality relates the edge conductance $\phi$ of a graph and the second smallest eigenvalue $\lambda_2$ of the Laplacian matrix. Recently, Olesker-Taylor and Zanetti discovered a Cheeger-type inequality $\psi^2 / \log |V| \lesssim \lambda_2^* \lesssim \psi$ connecting the vertex expansion $\psi$ of a graph $G=(V,E)$ and the maximum reweighted second smallest eigenvalue $\lambda_2^*$ of the Laplacian matrix. In this work, we first improve their result to $\psi^2 / \log d \lesssim \lambda_2^* \lesssim \psi$ where $d$ is the maximum degree in $G$, which is optimal assuming the small-set expansion conjecture. Also, the improved result holds for weighted vertex expansion, answering an open question by Olesker-Taylor and Zanetti.

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