On strong hyperbolicity for first order systems (Q1067096)

The author studies the strong hyperbolicity for first order hyperbolic systems \[ L(x,D)=-D_ 0+\sum^{d}_{j=1}A_ j(x)D_ j+B(x) \] where \(A_ j(x)\), B(x) are \(N\times N\) matrices with smooth entries defined near the origin in \(R^{d+1}\) with coordinates \(x=(x_ 0,x^ 1)=(x_ 0,x_ 1,...,x_ d)\) and \(D_ j=-i(\partial /\partial x_ j)\).