J. Vaillant, SYSTEMS OF PARTIAL-DIFFERENTIAL EQUATIONS AND CLASSES OF GEVREY, Comptes rendus de l'Academie des sciences. Serie 1, Mathematique, 320(12), 1995, pp. 1469-1474
We consider a square matrix h of analytic linear partial differential
operators; we define invariant conditions LG which depend only on h an
d which permit to determine the Gevrey classes where the Cauchy proble
m is well posed.