1. Departamento de Matematica Univ. de São Paulo, Caixa Postal 66281, São Paulo, SP, 05315-970 – Brazil, Brazil
2. Faculty of Mechanics and Mathematics, Kiev National, Taras Shevchenko Univ., Vladimirskaya Str., 64, 01033 Kiev, Ukraine
3. Faculty of Mechanics and Mathematics, Kharkov National, Karazin Univ., Svobody sq., 4, 61077 Kharkov, Ukraine