A dichotomy for D-rank 1 types in simple theories

Abstract

We prove a dichotomy for D-rank 1 types in simple theories that generalizes Buechler’s dichotomy for D-rank 1 minimal types in stable theories: every D-rank 1 type is either 1-based or part of its algebraic closure, defined by a single formula, almost contains a (non-algebraic) formula that belongs to a non-forking extension of the type. In addition we prove that a densely 1-based type of D-rank 1 is 1-based. We also observe that for a hypersimple unidimensional theory the existence of a non-algebraic stable type implies stability (and thus superstability).