Affiliation: | Mathematical Institute, University of Wroclaw pl. Grunwaldzki 2/4, 50-384 Wroclaw, Poland -- and -- Instituto de Matemáticas, UNAM Circuito Exterior, Ciudad Universitaria, 04510 México, D. F., México ; Mathematical Institute, University of Wroclaw pl. Grunwaldzki 2/4, 50-384 Wroclaw, Poland -- and -- Department of Mathematics and Statistics, University of Missouri-Rolla, Rolla, Missouri 65409-0020 |