2 00 1 Group analysis of hydrodynamic - type systems

We study point and higher symmetries for the hydrodynamic-type systems with two independent variables t and x with and without explicit dependence of the equations on t, x. We consider those systems which possess an infinite group of the hy-drodynamic symmetries, establish existence conditions for this property and, using it, derive linearizing transformations for these systems. The recursion operators for symmetries are obtained and used for constructing infinite series of exact solutions of the studied equations. Higher symmetries, i.e. the Lie-Bäcklund transformation groups, are also studied and the interrelation between the existence conditions for higher symmetries and recursion operators is established. More complete results are obtained for two-component systems, though n-component systems are also studied. In particular, we consider Hamiltonian and semi-Hamiltonian systems.