1. Department of Mathematics and Informatics, Faculty of Applied Sciences, University Politehnica of Bucharest, Splaiul Independentei 313, RO-060042 Bucharest, Romania
2. Academy of Romanian Scientists, Ilfov 3, RO-050044 Bucharest, Romania
3. Department of Mechanisms and Robot Theory, University Politehnica of Bucharest, Splaiul Independentei 313, RO-060042 Bucharest, Romania