Propagation of singularities for hyperbolic operators with multiple involutive characteristics (Q5903731)

The author extends previous result on propagation of \(C^{\infty}\)- singularities for involutive pseudo-differential operators of \textit{R. B. Melrose} and \textit{G. A. Uhlmann} [Duke Math. J. 46, 571-582 (1979; Zbl 0422.58026)]. The technique of the proof is that of Melrose-Uhlmann. In generality, it is analogous to a result of \textit{J. Sjöstrand} [Ann. Inst. Fourier 26, No.1, 141-155 (1976; Zbl 0317.58018)] where an ellipticity and Calderon's type of uniqueness conditions are replaced by a strictly hyperbolic's condition.