1. Mathematics, Brno University of Technology, Brno, Czech Republic, 616 00 (e-mail: diblik@feec.vutbr.cz)
2. Mathematics, Ben-Gurion University of the Negev, Department of Mathematics, Beer-Sheva 84105, Israel, Israel, 8463300 (e-mail: brznsky@cs.bgu.ac.il)
3. Department of Mathematics, Brno University of Technology, Faculty of Electrical Engineering and Communication, Department of Mathematics, 616 00 Brno, Brno, Czech Republic, 61600 (e-mail: svobodaz@feec.vutbr.cz)
4. Department of Mathematics, Brno University of Technology, Faculty of Electrical Engineering and Communication, Department of Mathematics, 616 00 Brno, Brno, Czech Republic, 61600 (e-mail: smarda@feec.vutbr.cz)