1. Instituto de Matemática, Universidade Federal do Rio de Janeiro, C. P. 68.530, CEP 21.945-970, Rio de Janeiro, RJ, Brazil
2. Department of Mathematics, University of Oklahoma, Norman, OK, USA
3. Department of Mathematics, Southern University of Science and Technology of China, Guangdong, China
4. Departamento de Geometria, Instituto de Matemática e Estatística, Universidade Federal Fluminense, Niterói, Brazil