Affiliation: | a Department of Mathematical Sciences, Aalborg University, Fredrik Bajers Vej 7G, 9220 Aalborg, Denmark b Dept. Theor. Phys., University of Bucharest, PO Box MG11, RO-76900 Bucharest, Romania c Inst. of Math. “Simion Stoilow” of the Romanian Academy, PO Box 1-764, RO-014700 Bucharest, Romania |