This article, which grew out of my lecture at the conference "Analysis and
Applications: A Conference in Honor of Elias M. Stein" in May 2011, is intended
to give an overview on a collection of results which have been obtained jointly
with I.I. Ikromov, and in parts also with M. Kempe, and at the same time to
give a kind of guided tour through the rather comprehensive proofs of the major
results that I shall address. All of our work is highly influenced by the
pioneering ideas developed by E.M. Stein.Comment: 44 pages, a picture