1. College of Mathematics and Statistics, Chongqing Jiaotong University, Chongqing 400074; and Chern Institute of Mathematics, Nankai University, Tianjin 300071, P. R. China
2. Departamento de Matemática, Universidade Federal de Paraíba, 58051-900 João Pessoa, PB, Brazil
3. Università degli Studi di Verona, Cà Vignal 2, Strada Le Grazie 15, 37134 Verona, Italy