[1] Univ Valladolid, Dep Fis Teor Atom & Opt, E-47071 Valladolid, Spain
[2] Univ Valladolid, EU Informat, Dep Informat, Segovia 40005, Spain
[3] Univ Valladolid, Dep Anal Matemat, E-47071 Valladolid, Spain
[4] Ankara Univ, Dept Phys, Fac Sci, TR-06100 Ankara, Turkey