First and foremost, I would like to express my sincere thanks to my thesis advisor, Assistant Professor Dr. Rattanaporn Wangkeeree for her invaluable help and constant encouragement throughout the course of this thesis. I am most grateful for her teaching and advice, not only the thesis methodologies but also many other methodologies in life. I would not have achieved this far and this thesis would not have been completed without all the support that I have always received from her.
In addition, I am grateful for the teachers in Department of Mathematics, Faculty of Science, Naresuan University and others for suggestions and all their help.
Finally, none of this would have been possible without the love and patience of my family. My immediate family to whom this dissertation is dedicated to, has been a constant source of love, concern and support throughout the period of this thesis.