This journal is archived by the American Mathematical Society. The master copy is available at http://www.ams.org/era/
Copyright 1997 American Mathematical Society
S. Kaliman
Department of Mathematics & Computer Science, University of Miami, Coral Gables, FL 33124
E-mail address: kaliman@paris-gw.cs.miami.edu
M. Koras
Institute of Mathematics, Warsaw University, Ul. Banacha 2, Warsaw, Poland
E-mail address: koras@mimuw.edu.pl
L. Makar-Limanov
Department of Mathematics & Computer Science, Bar-Ilan University, 52900 Ramat-Gan, Israel, and Department of Mathematics, Wayne State University, Detroit, MI 48202
E-mail address: lml@bimacs.cs.biu.ac.il; lml@math.wayne.edu
P. Russell
Department of Mathematics & Statistics, McGill University, Montreal, QC, Canada, and Centre Interuniversitaire, en Calcul Mathématique, Algébrique (CICMA)
E-mail address: russell@Math.McGill.CA
The first author was partially supported by an NSA grant