1. Faculty of Mathematics and Computer Science, Nicholas Copernicus University, ul. Chopina 12/18, 87–100 Toruń, Poland
2. Institute of Mathematics, Warsaw University, ul. Banacha 2, 00-989 Warszawa, Poland
3. Laboratoire R. Salem, UMR CNRS 6085, UFR Sciences, Université de Rouen, 76821 Mont Saint Aignan Cedex, France
4. Department of Mathematics, University of Tennessee, Knoxville, TN 37996–1300, USA