The authors of the present book share the view that groups and semigroups playa funda mental role in the structure of the complex systems which they are studying. A serious effort was made to implement this point of view by presenting the fundamental concepts pertaining to groups and semigroups before going into the various fields of application. The first two chapters are written in this spirit. The following seven chapters deal with groups in relation to specific systems and lead from basic notions to high-level applications. The systems under study are in all cases characterized by a high...

Matter may be divided into four phases or states: solid, liquid, gas and plasma (ionized gas). The termfluid has been used as a general name for the last three states: liquid, gas, and plasma because they may be deformed without applying any force, provided that the change of shape occurs very slowly. Furthermore, when a large number of small solid particles flow in a fluid, if the velocity of the fluid is sufficiently high, the behavior of such solids is similar to ordinary fluid. We may consider these solid particles as pseudofluid. Under proper conditions, we may treat fluid flow problems...