Affiliation: | 1.Faculty of Mathematics and Informatics, Sofia University, James Bourchier Boul. 5, 1126, Sofia, Bulgaria ;2.Institute of Mathematics and Informatics, Bulgarian Academy of Sciences, G.Bonchev str., bl. 8, 1113, Sofia, Bulgaria ;3.DIM–CMM, UMI CNRS 2807, FCFM, Universidad de Chile, Beauchef 851, Santiago, Chile ;4.Laboratoire de Mathématiques de Bretagne Atlantique (CNRS UMR 6205), Université de Bretagne-Occidentale (Brest), 6, avenue Victor Le Gorgeu, 29200, Brest, France ; |