[1] Russian State Univ Tourism & Serv, Inst Tourism & Hospitality, Kronstadt Blvd 32A, Moscow 125438, Russia
[2] Indiana Univ, Dept Phys, Bloomington, IN 47405 USA
[3] Univ Seville, Dept Geometria & Topol, C Tarfia S-N, E-41012 Seville, Spain
[4] Univ Seville, Dept Matemat Aplicada 1, Avda Reina Mercedes S-N, Seville, Spain