Affiliation: | Department of Mathematics, Rose-Hulman Institute of Technology, Terre Haute, Indiana 47803 ; Departamento de Matematicas, Fund. UNED, 28040 Madrid, Spain ; Departamento de Matematicas, Fund. UNED, 28040 Madrid, Spain ; Departamento de Algebra, Universidad Complutense de Madrid, 28040 Madrid, Spain ; Instytut Matematyki WSP, Chodkiewicza 30, 85-064 Bydgoszcz, Poland |