Intersection sets in AG(n,q) and a characterization of the hyperbolic quadric in PG(3,q) Original Research Article

Bruen proved that if A is a set of points in AG(n,q) which intersects every hyperplane in at least t points, then |A|⩾(n+t−1)(q−1)+1, leaving as an open question how good such bound is. Here we prove that, up to a trivial case, if t>((n−1)(q−1)+1)/2, then Bruenʹs bound can be improved. If t is equal to the integer part of ((n−1)(q−1)+1)/2, then there are some examples which attain such a lower bound. Somehow, this suggests the following combinatorial characterization: if a set S of points in PG(3,q) meets every affine plane in at least q−1 points and is of minimum size with respect to this property, then S is a hyperbolic quadric.