1. 1. Alekseevsky, D., Kriegl, A., Michor, P.W., Losik, M.: Choosing roots of polynomials smoothly. Israel J. Math. 105, 203–233 (1998)
2. 2. Bronšteĭn, M.D.: Smoothness of roots of polynomials depending on parameters. Sibirsk. Mat. Zh. 20, no. 3, 493–501, 690 (1979)
3. 3. Bronšteĭn, M.D.: The Cauchy problem for hyperbolic operators with characteristics of variable multiplicity. Trudy Moskov. Mat. Obshch. 41, 83–99 (1980)
4. 4. D'Ancona, P., Kinoshita, T., Spagnolo, S.: Weakly hyperbolic systems with Hölder continuous coefficients. J. Differ. Equations 203, no. 1, 64–81 (2004)
5. 5. D'Ancona, P., Spagnolo, S.: On pseudosymmetric hyperbolic systems. Ann. Sc. Norm. Super. Pisa, Cl. Sci., IV. Ser. 25, no. 1-2, 397–418 (1997)