Daniel Lorenz, Michael Reissig, “A generalized Levi condition for weakly hyperbolic Cauchy problems with coefficients low regular in time and smooth in space”, J. Pseudo-Differ. Oper. Appl., 10:1 (2019), 199
Seiiehiro WAKABAYASHI, “Singularities of solutions of the Cauchy problem for hyperbolic systems in Gevrey classes”, Jpn. j. math, 11:1 (1985), 157
