Preface

The text which follows is a preprint version of my paper. I am grateful to the editors of these Proceedings for their suggestion to publish it. I must explain here that the published version of the paper differs significantly from the text of the preprint version. Actually at the time when the preprint was written (1981) many technical points of this work were still quite cumbersome. For this reason I assumed that the length of the proof would not allow me to publish it in a journal. I was thinking about writing a book. But to begin with I decided to make a short version (a conspectus), with only sketches of proofs, and distributed it as a preprint.

Fortunately, at the time when the final version was written (1987), a number of mathematicians had already contributed much to the subject. Some essential technical points, especially related to the construction of the product in KK-theory, were significantly simplified. So I benefited greatly from the existing publications which allowed me to publish the final text as a journal paper.

However, I find it reasonable to publish now also the preprint version. Actually there are two reasons. First of all, the exposition of the published version contains, in addition to technical improvements, also some generalizations of the main constructions of the preprint. So some people find it more difficult to read the published version.