# Research: Number Theory

## Summer School 2021: Applications of Expander Graphs to Number Theory and Computer Science, May 24-28.

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From May 24 to 28, 2021, the University of North Carolina Greensboro will host the UNCG Summer School in Computational Number Theory and Algebra: **Applications of Expander Graphs to Number Theory and Computer Science.**

Expanders are graphs satisfying very strong connectivity properties. There are precise definitions, but, roughly speaking, every “small” set of vertices in an expander has “many” neighbors outside that set. We will introduce expanders and then explore the many connections they have to topics in number theory and computer science.

The school will run in morning and afternoon zoom sessions for the five days between May 24 and May 28, 2021. In each session, one of our well-known speakers will give a lecture. Then everyone will break into small zoom groups to work on problem sets together with mentors. All problems are aimed at increasing the students’ understanding of the material by working with it. At the end of each session, the whole school will reconvene to talk over the results and for further insight. The talks early in the week will introduce the students to the subject and those later in the week cover related areas of current research and unsolved problems.

This school is targeted primarily to early stage graduate students in mathematics with an interest in number theory.

### Speakers

- Irit Dinur, Weizman Institute of Science
- Alex Kontorovich, Rutgers University
- Emmanuel Kowalski, ETH Zürich

### Topics

Definitions – combinatorial expansion, laplacian spectrum, random walks.

Constructions – randomized constructions, Cayley graphs, Ramanujan graphs, the Zig-Zag Product.

Applications – survey, sieves in discrete groups, Apollonian circle packings, arithmetic geometry, error correcting codes.

### Schedule

The school will be held via zoom.

Monday May 24 to Friday May 28, 2021.

Mornings: 9 AM – 11:30 AM, Eastern Standard Time

Afternoons: 1 PM – 3:30 PM, Eastern Standard Time

Schedule per session: 50 minutes of lecture, 80 minutes of problem solving in small mentored groups, 20 minutes of large group discussion

### Application

Interested students should complete the online application form and have a letter of reference sent via email to uncg.expander.school.2021@gmail.com by **Sunday April 25, 2021**. We will respond with decisions by May 8, 2021.

### Organizers

Talia Fernos, Sebastian Pauli, Filip Saidak, Clifford Smyth, Brett Tangedal, Dan Yasaki

### Acknowledgements

The organizers, speakers, and participants are all grateful for the support from UNCG and the NSF (DMS-1802448) that makes this school possible.