1. Department of Applied Mathematics, National University of Science and Technology POLITEHNICA Bucharest, Bucharest, Romania
2. Center for Research and Training in Innovative Techniques of Applied Mathematics in Engineering, National University of Science and Technology POLITEHNICA Bucharest, Bucharest, Romania
3. Faculty of Mathematics and Computer Science, University of Bucharest, Bucharest, Romania
4. Faculty of Applied Sciences, National University of Science and Technology POLITEHNICA Bucharest, Bucharest, Romania