Well-posedness results for hyperbolic operators with coefficients oscillating in time

In the present paper, we consider second order strictly hyperbolic linear operators of the form $Lu\,=\,\d_t^2u\,-\,\div\big(A(t,x)\nabla u\big)$, for $(t,x)\in[0,T]\times{\mathbb R}^n$. We assume the coefficients of the matrix $A(t,x)$ to be smooth in time on $\,]0,T]\times{\mathbb R}^n$, but rapidly oscillating when $t\to0^+$; they match instead minimal regularity assumptions (either Lipschitz or log-Lipschitz regularity conditions) with respect to the space variable. Correspondingly, we prove well-posedness results for the Cauchy problem related to $L$, either with no loss of derivatives (in the Lipschitz case) or with a finite loss of derivatives, which is linearly increasing in time (in the log-Lipschitz case).