Affiliations: Jana Kopfová, Petra Nábělková, Mathematical Institute of the Silesian University, Na Rybníčku 1, 746 01 Opava, Czech Republic, e-mail: jana.kopfova@math.slu.cz, petra.nabelkova@math.slu.cz; Dmitrii Rachinskii, Department of Mathematical Sciences, University of Texas at Dallas, 800 W. Campbell Road, Richardson, Texas 75080-3021, USA, e-mail: dmitry.rachinskiy@utdallas.edu