1 Department of Engineering Sciences and Mathematics, Lulea University of Technology, Lulea, SE-97187, Sweden
2 Narvik University College, PO Box 385, Narvik, NO 8505, Norway
3 Department of Mathematical Analysis and Function Theory, Peoples' Friendship University of Russia, Miklukho-Maklay 6, Moscow, 117198, Russian Federation
4 Steklov Mathematical Institute, Russian Academy of Sciences, Gubkina 8, Moscow, 119991, Russian Federation