[1] Univ. Lyon, Université Claude-Bernard Lyon1, CNRS UMR 5208, Institut Camille Jordan, 43, Bd. du 11 Novembre 1918, Villeurbanne
[2] Department of Mathematics and Statistics, University of Calgary, Calgary, AB
[3] Departamento de Matematica, Universidade Federal de Minas Gerais (UFMG), Av. Antonio Carlos, 6627, Caixa Postal 702, Região Pampulha, MG, Belo Horizonte