Introduction

Chapter 1 is dedicated to the origins of the “ S -variable approach.” We trace the contributions of independent authors that participated to establish the fundamentals and justify our choice for the denomination “ S -variable approach.” This attentive detailed study is the occasion to show several interpretations of the S -variables with respect to technical results such as Finsler’s lemma and elimination lemma. The chapter concludes with the brief exposure of all the problems to be tackled in the remaining part of the book, justifying the importance of these selected problems.