1. Department of Mathematics and Physics, The University of Campania ``Luigi Vanvitelli'', Viale Lincoln 5, 81100 Caserta, Italy
2. Czech Technical University, Faculty of Nuclear Sciences and Physical Engineering, Břehová 7, 11519 Prague 1, Czech Republic
3. Institute of Rock Structure and Mechanics, Czech Academy of Sciences, V Holešovičkách 41, 18209 Prague 8, Czech Republic