On Frobenius incidence varieties of linear subspaces over finite fields

We define Frobenius incidence varieties by means of the incidence relation of Frobenius images of linear subspaces in a fixed vector space over a finite field, and investigate their properties such as supersingularity, Betti numbers and unirationality. These varieties are variants of the Deligne-Lusztig varieties. We then study the lattices associated with algebraic cycles on them. We obtain a positive-definite lattice of rank 84 that yields a dense sphere packing from a 4-dimensional Frobenius incidence variety in characteristic 2.Comment: 24 pages, no figures; Introduction is changed. New references are adde

Transcendental lattices and supersingular reduction lattices of a singular K3K3 surface

A (smooth) K3 surface X defined over a field k of characteristic 0 is called singular if the N\'eron-Severi lattice NS (X) of X over the algebraic closure of k is of rank 20. Let X be a singular K3 surface defined over a number field F. For each embedding \sigma of F into the complex number field, we denote by T(X^\sigma) the transcendental lattice of the complex K3 surface X^\sigma obtained from X by \sigma. For each prime ideal P of F at which X has a supersingular reduction X_P, we define L(X, P) to be the orthogonal complement of NS(X) in NS(X_P). We investigate the relation between these lattices T(X^\sigma) and L(X, P). As an application, we give a lower bound of the degree of a number field over which a singular K3 surface with a given transcendental lattice can be defined.Comment: 40 pages, revised version, to appear in Transactions of the American Mathematical Societ