ИСТИНА

We show that when modeling some phenomena of various nature nonstrictly hyperbolic systems of nonlinear equations of a special form or their parabolic regularizations arise. Such systems can be studied in a similar way. As examples, we discuss several models for which oscillatory processes are natural: a model of electron plasma, a model of a stratified fluid, and a model of movement of a fluid in a closed tube with reacting walls (a prototype of the blood circulatory system). In all these cases, the oscillations can be regular or blow up with time. In addition, traveling waves can exist in all these systems.