Department of Mathematics, University of A Coruña, School of Computer Science, Campus de Elviña, s/n, 15071 A Coruña, Spain.|Department of Mathematics, University of A Coruña, School of Computer Science, Campus de Elviña, s/n, 15071 A Coruña, Spain.|Department of Mathematics, University of A Coruña, School of Computer Science, Campus de Elviña, s/n, 15071 A Coruña, Spain.