In this paper we consider the Cauchy problem for higher order weakly hyperbolic equations. We assume that the principal symbol depends only on one space variable and the characteristic roots $\tau_j$ verify the inequality \[\tau_j^2(x) + \tau_k^2(x) \le M \bigl(\tau_j(x)-\tau_k(x)\bigr)^2\] for some constant $M$ independent of $x$. We prove that if the lower order terms verify a suitable Levi condition, the Cauchy problem is well-posed in $\mathcal{ C}$-infinity.
Categories: math.AP