boundary problem for hyperbolic systems of the equations in private derivative; the group analysis; plasticity; exact solutions of differential equations.

The hyperbolic system of plane ideal plasticity equations under the Saint-Venant - Mises’ yield criterion is considered. Characteristics on this system are deformed under the action of point transformations committed by the group that allows to construct new analytic solutions. The mechanical sense of obtained characteristic fields is discussed. The general algorithm of solution transformation of two quasilinear homogeneous equations hyperbolic system of two independent variables is proposed.