Institution: | (1) Department of Mathematics, Odessa State University, Petr Veliki St. 2, 27000 Odessa, UKRAINE;(2) Instituto Superior, Minero-Metalúrgico de Moa, Moa, CUBA;(3) Present address: Departamento de Matemáticas, CINVESTAV del I.P.N., Apartado Postal 14-740, 07000 México, D.F., MEXICO |