2022 Theses Doctoral

Geometry of q-bic Hypersurfaces

Cheng, Raymond

Traditional algebraic geometric invariants lose some of their potency in positive characteristic. For instance, smooth projective hypersurfaces may be covered by lines despite being of arbitrarily high degree. The purpose of this dissertation is to define a class of hypersurfaces that exhibits such classically unexpected properties, and to offer a perspective with which to conceptualize such phenomena.

Specifically, this dissertation proposes an analogy between the eponymous q-bic hypersurfaces—special hypersurfaces of degree q+1, with q any power of the ground field characteristic, a familiar example given by the corresponding Fermat hypersurface—and low degree hypersurfaces, especially quadrics and cubics. This analogy is substantiated by concrete results such as: q-bic hypersurfaces are moduli spaces of isotropic vectors for a bilinear form; the Fano schemes of linear spaces contained in a smooth q-bic hypersurface are smooth, irreducible, and carry structures similar to orthogonal Grassmannian; and the intermediate Jacobian of a q-bic threefold is purely inseparably isogenous to the Albanese variety of its smooth Fano surface of lines.