This section introduces some basic concepts of the theory of linear systems. For a more complete and rigorous treatment, the reader should consult a textbook such as [Oppenheim & Willsky 1983]. Digital signal processing is based on the same concepts but the mathematical formulations are different for discrete (sampled) signals [Oppenheim & Schafer 1975,Scherbaum 1996,Plesinger et al. 1996]. Readers who are familiar with the mathematics may proceed to section 2.7.