Multiplicity-free Schubert calculus

Multiplicity-free algebraic geometry is the study of subvarieties Y⊆X with the "smallest invariants" as witnessed by a multiplicity-free Chow ring decomposition of [Y] ε A^-(X) into a predetermined linear basis. This paper concerns the case of Richardson subvarieties of the Grassmannian in terms of the Schubert basis. We give a nonrecursive combinatorial classification of multiplicity-free Richardson varieties, i.e., we classify multiplicity-free products of Schubert classes. This answers a question of W. Fulton.