1. Faculty of Applied Sciences, National University of Science and Technology Politehnica Bucharest, 060042 Bucharest, Romania
2. Academy of Romanian Scientists, 54 Splaiul Independentei, 050094 Bucharest, Romania
3. Fundamental Sciences Applied in Engineering-Research Center, National University of Science and Technology Politehnica Bucharest, 060042 Bucharest, Romania
4. Department of Mathematics, Faculty of Science and Letters, Aksaray University, Aksaray 68100, Turkey