1. State Key Laboratory for Geo-Mechanics and Deep Underground Engineering, School of Mechanics and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, P. R. China
2. Institute of Engineering, Polytechnic of Porto, Department of Electrical Engineering, Rua Dr. António Bernardino de Almeida, 4249-015 Porto, Portugal
3. Department of Mathematics, Cankya University, Ogretmenler Cad. 14, Balgat-06530, Ankara, Turkey
4. Institute of Space Sciences, Magurele-Bucharest, Romania