This is the second volume of the English translation of the original manuscript of the author published earlier in Greek, and it presents lecture notes of a series of seminars given by the author in 1984. This volume contains two extended lectures. The first lecture \(D\) discusses various applications of the Huygens’s principle and classical wave propagation. The lecture \(E\) presents the background of the theory and different applications of linear differential equations. It also gives an introduction into the Liouville-Radon transform.