Well-posedness for general 2 x 2 conservation laws

We consider the Cauchy problem for a strictly hyperbolic $2\times 2$ system of conservation laws in one space dimension % $$ u_t+[F(u)]_x=0,\qquad\qquad u(0,x)=\bar u(x), \tag 1 $$ % which is neither linearly degenerate nor genuinely non-linear. We make the following assumption on the characteristic fields. If $r_i(u), \ i=1,2,$ denotes the $i$-th right eigenvector of $DF(u)$ and $\lambda_i(u)$ the corresponding eigenvalue, then the set $\{u : \nabla \lambda_i \cdot r_i (u) = 0\}$ is a smooth curve in the $u$-plane that is transversal to the vector field $r_i(u).$ Systems of conservation laws that fulfill such assumptions arise in studying elastodynamics or rigid heat conductors at low temperature. For such systems we prove the existence of a closed domain \ $\Cal D \subset L^1,$ \ containing all functions with sufficiently small total variation, and of a uniformly Lipschitz continuous semigroup $S:{\Cal D} \times [0,+\infty)\rightarrow \Cal D$ with the following properties. Each trajectory \ $t \mapsto S_t \bar u$ \ of $S$ is a weak solution of (1). Viceversa, if a piecewise Lipschitz, entropic solution $u= u(t,x)$ of (1) exists for $t \in [0,T],$ then it coincides with the trajectory of $S$, i.e. $u(t,\cdot) = S_t \bar u.$ This result yields the uniqueness and continuous dependence of weak, entropy-admissible solutions of the Cauchy problem (1) with small initial data, for systems satysfying the above assumption.