“…Solutions of a piston problem were analyzed in [15,Chapter 5], [17,Section 7.4.7], [22,23]. For fixed initial values U 1 , U 2 , the whole u 1 , u 2 plane is divided into domains in such a way that, if boundary values u * 1 , u * 2 belong to one of them, then a definite combination of Riemann waves and shocks gives the solution.…”