Cauchy problem for effectively hyperbolic operators with triple characteristics

We study a class of third-order effectively hyperbolic operators P with triple characteristics . V. Ivrii introduced the conjecture that every effectively hyperbolic operator is strongly hyperbolic, that is the Cauchy problem for P + Q is locally well posed for any lower-order terms Q . For operators with triple characteristics, this conjecture was established in the case when the principal symbol of P admits a factorization as a product of two symbols of principal type. A strongly hyperbolic operator in G could have triple characteristics in G only for t = 0 or for t = T . The operators that we investigate have a principal symbol which in general is not factorizable and we prove that these operators are strongly hyperbolic if T is small enough.