Institution: | a Institute of Mathematics, Warsaw University of Technology, Pl. Politechniki 1, 00-661, Warsaw, Polandf1E-mail: strozyna@mini.pw.edu.plf1b Institute of Mathematics, University of Warsaw, ul. Banacha 2, 02-097, Warsaw, Polandf2E-mail: zoladek@mimuw.edu.plf2 |