Introduction

Many practical situations lead to the necessity of studying multi-stage systems, i.e., systems in which the processing of each job consists of several sequential stages, at each of which the job is processed on a certain machine or, in a more general case, on a certain set of machines. For example, manufacturing a workpiece usually includes several sequential operations, each of which is performed on some of the machine tools available at the workshop; a teacher gives lectures one after another for several specific groups of students; the process of preparing a book for publication involves stages such as writing, reviewing, editing, printing, etc.