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

In the present paper, we consider second order strictly hyperbolic linear operators of the form $Lu={\mathrm{\partial }}_t^{2}u-\mathrm{d}\mathrm{i}\mathrm{v}(A(t,x)\mathrm{\nabla }u)$, 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 $t\to 0^{+}$; 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).