Well - Rounded Integral Lattices in Dimension Two

A lattice is called well-rounded if its minimal vectors span the corresponding Eucildean space. In this paper we completely describe well-rounded full-rank sublattices of Z 2 , as well as their determinant and minima sets. We show that the determinant set has positive density, deriving an explicit lower bound for it, while the minima set has density 0. We also produce formulas for the number of such lattices with a fixed determinant and with a fixed minimum. These formulas are related to the number of divisors of an integer in short intervals and to the number of its representations as a sum of two squares. We investigate the growth of the number of such lattices with a fixed determinant as the determinant grows, exhibiting some determinant sequences on which it is particularly large. To this end, we also study the behaviour of the associated zeta function, comparing it to the Dedekind zeta function of Gaussian integers and to the Solomon zeta function of Z 2. Our results extend automatically to well-rounded sublattices of any lattice AZ 2 , where A is an element of the real orthogonal group O 2 (R).