1. Institute of Mathematics, NAS of Ukraine, Tereschenkivska str. 3, Kiev, 01601, Ukraine
2. Cybernetics Department, Kiev Taras Shevchenko University, Volodymyrska str. 64, Kiev, 01033, Ukraine
3. Institute of Mathematics, Leipzig University, Johannisgasse 26, Leipzig, 04103, Germany
4. Department of Mathematical Sciences, Chalmers University of Technology and University of Gothenburg, Gothenburg, SE 412-96, Sweden