## Lethbridge Number Theory and Combinatorics Seminar: Mohammad Bardestani

- Date: 11/23/2015
- Time: 12:00

*University of Ottawa*

University of Lethbridge

Isotropic quadratic forms and the Borel chromatic number of quadratic graphs

For a field F and a quadratic form Q defined on an n-dimensional vector space V over F, let GQ, called the quadratic graph associated to Q, be the graph with the vertex set V where vertices v, w in V form an edge if and only if Q(v-w) = 1. Quadratic graphs can be viewed as natural generalizations of the unit-distance graph featuring in the famous Hadwiger-Nelson problem. In the present talk, we will prove that for a localĀ field F of characteristic zero, the Borel chromatic number of GQ is infinite if and only if Q represents zero non-trivially over F. The proof employs a recent spectral bound for the Borel chromatic number of Cayley graphs, combined with an analysis of certain oscillatory integrals over local fields. As an application, we will also answer a variant of question 525 proposed in the 22nd British Combinatorics Conference 2009.