Compatibly split subvarieties of the Hilbert scheme of points in the plane

Abstract

Let k be an algebraically closed field of characteristic p>2. By a result of Kumar and Thomsen, the standard Frobenius splitting of the affine plane induces a Frobenius splitting of the Hilbert scheme of n points in the plane. In this thesis, we investigate the question, "what is the stratification of the Hilbert scheme of points in the plane by all compatibly Frobenius split subvarieties?" We provide the answer to this question when n is at most 4 and we give a conjectural answer when n=5. We prove that this conjectural answer is correct up to the possible inclusion of one particular one-dimensional subvariety of the Hilbert scheme of 5 points, and we show that this particular one-dimensional subvariety is not compatibly split for at least those primes p between 3 and 23. Next, we restrict the splitting of the Hilbert scheme of n points in the plane (now for arbitrary n) to the affine open patch U_<x,y^n> and describe all compatibly split subvarieties of this patch and their defining ideals. We find degenerations of these subvarieties to Stanley-Reisner schemes, explicitly describe the associated simplicial complexes, and use these complexes to prove that certain compatibly split subvarieties of U_<x,y^n> are Cohen-Macaulay.