[1] Univ Concepcion, Dept Matemat, Concepcion, Chile
[2] Leibniz Univ Hannover, Inst Algebra Zahlentheorie & Diskrete Math, D-30167 Hannover, Germany
[3] Univ Tubingen, Math Inst, D-72076 Tubingen, Germany
[4] Univ Concepcion, Dept Matemat, Concepcion, Chile