Affiliation: | (1) Department of Mathematics, Carnegie mellon University, 15213 Pittsburgh, PA, U.S.A.;(2) Instytut Matematyki, Uniwersitet a. Mickiewicza, UL. Matejki 48/49, PL-60769 Pozna, Poland;(3) Present address: Institute for Mathematics and its applications, University of Minnesota, 55455 Minneapolis, MN, U.S.A. |