Uniqueness of weak solutions to systems of conservation laws

Consider a strictly hyperbolic n x n system of conservation laws in one space dimension: u(t) + F(u)(x) = 0. (*) Relying on the existence of the Standard Riemann Semigroup generated by (*), we establish the uniqueness of entropy-admissible weak solutions to the Cauchy problem, under a mild assumption on the variation of u along spacelike segments.