[1] ST PETERSBOURG RAILWAY ENGN INST, DEPT MATH, ST PETERSBURG, RUSSIA
[2] UNIV CANTABRIA, FAC CIENCIAS, DEPT FIS APLICADA, E-39005 SANTANDER, SPAIN
[3] ST PETERSBOURG TELECOMMUN INST, DEPT PHYS, ST PETERSBURG 191005, RUSSIA
[4] INST PHYS NUCL LYON, DIV PHYS THEOR, F-91406 ORSAY, FRANCE