1. Department of Mathematics, Faculty of Science, University of Malta, Msida MSD 2080, Malta. david.buhagiar@um.edu.mt; emanuel.chetcuti@um.edu.mt
2. Institute of Mathematics, Slovak Academy of Sciences, Štefánikova 49, SK-814 73 Bratislava, Slovakia
3. Department of Algebra and Geometry, Faculty of Sciences, Palacký University, tř. 17. listopadu 1192/12, CZ-771 46 Olomouc, Czech Republic. dvurecen@mat.savba.sk