Furstenberg sets and Furstenberg schemes over finite fields

Algebra Seminar
Friday, March 27, 2015 - 15:05
1 hour (actually 50 minutes)
Skiles 006
University of Wisconsin, Madison

Useful background:The paper I’m discussing:&nbsp; <a href="http://arxiv.org/abs/1502.03736" title="http://arxiv.org/abs/1502.03736">http://arxiv.org/abs/1502.03736</a>Terry Tao’s blog post on Dvir’s theorem:&nbsp; <a href="https://terrytao.wordpress.com/2008/03/24/dvirs-proof-of-the-finite-fiel... title="https://terrytao.wordpress.com/2008/03/24/dvirs-proof-of-the-finite-fiel... earlier paper with Terry and Richard Oberlin about Kakeya restriction over finite fields:&nbsp; <a href="http://arxiv.org/abs/0903.1879" title="http://arxiv.org/abs/0903.1879">http://arxiv.org/abs/0903.1879</a>

The study of extremal configurations of points and subspaces sits at the boundary between combinatorics, harmonic analysis, and number theory; since Dvir’s 2008 resolution of the Kakeya conjecture over finite fields, it has been clear that algebraic geometry is also part of the story.We prove a theorem of Kakeya type for the intersection of subsets of n-space over a finite field with k-planes. Let S be a subset of F_q^n with the "k-plane Furstenberg property": for every k-plane V, there is a k-plane W parallel to V which intersects S in at least q^c points. We prove that such a set has size at least a constant multiple of q^{cn/k}. The novelty is the method; we prove that the theorem holds, not only for subsets of the plane, but arbitrary 0-dimensional subschemes, and reduce the problem by Grobner methods to a simpler one about G_m-invariant non-reduced subschemes supported at a point. The talk will not assume that everyone in the room is an algebraic geometer.  It will, however, try to convince everyone in the room that it can be useful to be an algebraic geometer.This is joint work with Daniel Erman.