1. UNIVERSITY OF BORDEAUX, LABRI UMR5800, F-33405 Talence, France; CNRS, LABRI, UMR5800 F-33405 TALENCE FRANCE
2. DEPARTMENT OF MATHEMATICS; UNIVERSITY OF JOHANNESBURG; AUCKLAND PARK 2006 SOUTH AFRICA
3. UNIVERSITY OF MONTPELLIER, LIRMM; UMR5506, MONTPELLIER FRANCE