Algebraic Methods in Discrete Analogs of the Kakeya Problem

We prove the joints conjecture, showing that for any $N$ lines in ${\Bbb R}^3$,thereareatmost$O(N^{{3 \over 2}})$ points at which 3 lines intersect non-coplanarly. We also prove a conjecture of Bourgain showing that given $N^2$ lines in ${\mathbb{R}}^3$ so that no $N$ lines lie in the same plane and so that each line intersects a set $P$ of points in at least $N$ points then the cardinality of the set of points is $\mathrm{\Omega }(N^3)$. Both our proofs are adaptations of Dvir’s argument for the finite field Kakeya problem.