Stability of $L^\infty$ Solutions for Hyperbolic Systems with Coinciding Shocks and Rarefactions

We consider a hyperbolic system of conservation laws \[ \left\{ \begin{array}{c} u_t + f(u)_x = 0, \\ u(0,\cdot) = u_0, \end{array} \right. \] where each characteristic field is either linearly degenerate or genuinely nonlinear. Under the assumption of coinciding shock and rarefaction curves and the existence of a set of Riemann coordinates w, we prove that there exists a semigroup of solutions $u(t) = \mathcal{S}_t u_0$, defined on initial data $u_0 \in L^\infty$. The semigroup $\mathcal{S}$ is continuous w.r.t. time and the initial data $u_0$ in the $L^1_{{\rm loc}}$ topology. Moreover, $\mathcal{S}$ is unique and its trajectories are obtained as limits of wave front tracking approximations.