Many people contributed in many ways to make my thesis possible. I would like to invest this opportunity to express my gratitude to them. First and foremost I am extremely grateful to my supervisors, Dr. Márton Naszódi and Dr. Zsolt Lángi for their appreciable advice, important comments, continuous support, and encouragement during my studies. I wish to thank all the staff members of the Institute of Mathematics for their help especially Dr. István Ágoston and Ms. Klaudia Szalay. I would like to thank the Tempus Public Foundation (TPF), Stipendium Hungaricum program, and University of Thi-Qar for their support.Finally, I would like to express my heartfelt gratitude to my father, mother, wife, and children, who endured so much for me.