[1] Univ Bordeaux, IMB, 351 Cours Liberat, F-33405 Talence, France
[2] CNRS, 351 Cours Liberat, F-33405 Talence, France
[3] Univ Valle, Dept Matemat, Calle 13 100-00, Cali 25360, Colombia
[4] Univ Witwatersrand, Sch Math, Johannesburg, South Africa
[5] King Abdulaziz Univ, Res Grp Algebra Struct & Applicat, Jeddah, Saudi Arabia
[6] UNAM, Ctr Ciencias Matemat, Morelia, Michoacan, Mexico