Affiliation: | (1) Department of Mathematics and Statistics, Concordia University, 1455 de Maisonneuve Blvd West, Montreal, H3G 1M8, QC, Canada;(2) Institute of Mathematics and Informatics, Bulgarian Academy of Science, 1113 Sofia, Bulgaria;(3) Department of Mathematics and Statistics, Memorial University of Newfoundland, St. John’s, NL, A1C 5S7, Canada |