n expansion Appendix S (Chapter 20) Computation of inverse Fourier transform in the factorization of N = 21 through Shor's algorithm Appendix T (Chapter 20) Modular arithmetic and Euler's theorem Appendix U (Chapter 21) Klein's inequality Appendix V (Chapter 21) Schmidt decomposition of joint pure states Appendix W (Chapter 21) State purification Appendix X (Chapter 21) Holevo bound Appendix Y (Chapter 25) Polynomial byte representation and modular multiplication Index Foreword It is always a great opportunity and pleasure for a professor to introduce a new text- book. This one is especially unusual, in a sense that, first of all, it concerns two fields, namely, classical and quantum information theories, which are rarely taught altogether with the same reach and depth. Second, as its subtitle indicates, this textbook primar- ily addresses the telecom scientist. Being myself a quantum-mechanics teacher but not being conversant with the current Telecoms paradigm and its community expecta- tions, the task of introducing such a textbook is quite a challenge. Furthermore, both subjects in information theory can be regarded by physicists and engineers from all horizons, including in telecoms, as essentially academic in scope and rather difficult to reconcile in their applications. How then do we proceed from there? I shall state, firsthand, that there is no need to convince the reader (telecom or physi- cist or both) about the benefits of Shannon's classical theory. Generally unbeknown to millions of telecom and computer users, Shannon's principles pervade all applications concerning data storage and computer files, digital music and video, wireline and wire- less broadband communications altogether. The point here is that classical information theory is not only a must to know from any academic standpoint; it is also a key to understanding the mathematical principles underlying our information society. Shannon's theory being reputed for its completeness and societal impact, the telecom engineer (and physicist within!