1. Department of Applied Mathematics, Illinois Institute of Technology, 10 West 32nd Str, Bld E1, Room 208, Chicago, IL 60616, USA
2. Department of Mathematics, University of Southern California, 3620 S. Vermont Avenue, KAP 108, Los Angeles, CA 90089, USA
3. Department of Mathematics, Faculty of Applied Sciences, University of West Bohemia, Univerzitní 22, 306 14 Plzeň, Czech Republic