1. Department of Mathematical Sciences, Aalborg University, Skjernvej 4A, 9220 Aalborg, Denmark
2. “Simion Stoilow” Institute of Mathematics of the Romanian Academy, P. O. Box 1-764, RO-014700, Bucharest, Romania
3. Laboratoire Européen Associé, CNRS Franco-Roumain Math-Mode, Bucharest, Romania