1. Institut für Allgemeine Mechanik, RWTH Aachen, Templergraben 64, D-52056, Aachen, Germany
2. Technical University of Łódź, Department of Automatics and Biomechanics, 1/15 Stefanowski St., PL-90-924, Łódź, Poland
3. National Metallurgical Academy of Ukraine, Department of Higher Mathematics, Gagarina 23, UA-49005, Dnepropetrovsk, Ukraine