Functional Equations (FEs) are a vaiable branch of mathematics. They arise in many applications and models. They can be either of one variable or many variables. Recently, queuing systems have been used to model numerous computers and communication systems. Queuing systems may be simple such as that which contains one server, and may not be simple such as that which contains more than one server. In some applications, the system which contains a queue or more interfering with one another, the equations characterizing these systems are functional equations. In this Book, we get an exact...