
ACKNOWLEDGMENT
This dissertation has been completed at the Faculty of Mathematics and In-
formatics, Hanoi National University of Education, under the guidance of Associate
Professor Le Van Hien. Let me take this opportunity to express my sincere thanks
to my supervisor, Associate Professor Le Van Hien, for his enlightening guidance, in-
sightful ideas, and endless support during my candidacy at Hanoi National University
of Education. His rigorous research ethics, diligent work attitude, and wholehearted
dedication to his students have been an inspiration to me and will influence me forever.
The author is grateful to Professor Cung The Anh, Associcate Professor Tran
Dinh Ke and other members of the weekly seminar at the Division of Mathemati-
cal Analysis, Faculty of Mathematics and Informatics, Hanoi National University of
Education, for their encouragement, valuable discussions and comments. I am also
grateful to Hong Duc University and the Division of Mathematics and Teaching meth-
ods, Faculty of Natural Science, for unlimited help and support during the time of my
postgraduate study.
I would like to express my gratitude to my parents for the encouragement, endless
love, and unconditional support they have been giving me. Last but not least, I am
indebted to my beloved wife, Mrs Le Thi Dao, our lovely son, Le Huy Nghia Linh, and
beautiful daughters, Le Thi Thu Hang, Le Thu Khanh Ngoc, who always trust and
stay beside me. None of this would have been possible without their continuous and
unconditional love, kindness and comfort through my journey.
The author
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