1. 1Department of Applied Mathematics, University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain
2. 2Department of Applied Mathematics, University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain
3. 3Institute of Mathematics and Computer Science, The John Paul II Catholic University of Lublin, Al. Raclawickie 14, 20-950 Lublin, Poland; and Institute of Mathematics, NAS of Belarus, 11 Surganov Str., 20072 Minsk, Belarus
4. 4Belarusian State University, 4 Nezavisimosti avenue, 220030 Minsk, Belarus