Earlier, in Chapter 1, we studied the behavior of discontinuities and constructed solutions of one-dimensional gas dynamic equations and some other systems of equations. In the case of small-amplitude waves described by arbitrary hyperbolic systems of equations, which represent conservation laws, the generalization of the standard classical behavior of the Riemann waves, discontinuities, and self-similar solutions is ensured by the Lax theorems (Lax 1957, see Chapter 1).