Cicognani, M. and Hirosawa, F. 2008. On the Gevrey well-posedness for second order strictly hyperbolic Cauchy problems under the influence of the regularity of the coefficients. MATHEMATICA SCANDINAVICA. 102, 2 (Jun. 2008), 283–304. DOI:https://doi.org/10.7146/math.scand.a-15063.