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X. Gui
4 Department of Mathematics,
Guizhou UniversityChina
M. Fečkan
Department of Mathematical Analysis and Numerical Mathematics Faculty of Mathematics, Physics and Informatics,
Comenius University in Bratislava,
Mathematical Institute of Slovak Academy of Sciences
Bratislava,Bratislava
J. R. Wang
Department of Mathematics,
Guizhou University GuiyangChina