Expansive subdynamics for algebraic Z^d-actions

A general framework for investigating topological actions of Zd on compact metric spaces was proposed by Boyle and Lind in terms of expansive behavior along lower-dimensional subspaces of Rd. Here we completely describe this expansive behavior for the class of algebraic Zd-actions given by commuting automorphisms of compact abelian groups. The description uses the logarithmic image of an algebraic variety together with a directional version of Noetherian modules over the ring of Laurent polynomials in several commuting variables. We introduce two notions of rank for topological Zd-actions, and for algebraic Zd-actions describe how they are related to each other and to Krull dimension. For a linear subspace of Rd we define the group of points homoclinic to zero along the subspace, and prove that this group is constant within an expansive component.