1. Sam Houston State University, Department of Mathematics and Statistics, Box 2206, Huntsville, TX 77341-2206, USA
2. Departamento de Álgebra and CITIC-UGR, Universidad de Granada, E-18071 Granada, España
3. Tufts University, Department of Mathematics, 503 Boston Avenue, Bromfield-Pearson, Room 103, Medford, MA 02155, USA
4. Dipartimento di Matematica e Informatica, Viale A. Doria, 6 – I 95125, Catania, Italia