1. 1. Agliardi, R., Cicognani, M.: The Cauchy problem for a class of Kovalevskian pseudo-differential operators. Proc. Amer. Math. Soc. 132, 841-845 (2004)
2. 2. Cicognani, M., Colombini, F.: Sharp regularity of the coefficients in the Cauchy Problem for a class of evolution equations. Differential Integral Equations 16, 1321-1344 (2003)
3. 3. Cicognani, M., Colombini, F.: Modulus of continuity of the coefficients and loss of derivatives in the strictly hyperbolic Cauchy problem. J. Differential Equations 221, 143-157 (2006)
4. 4. Colombini, F., De Giorgi, E., Spagnolo, S.: Sur les équations hyperboliques avec des coefficients qui ne dépendent que du temps. Ann. Scuola Norm. Sup. Pisa Cl. Sci. 6, 511-559 (1979)
5. 5. Colombini, F., Lerner, N.: Hyperbolic operators with non-Lipschitz coefficients. Duke Math. J. 77, 657-698 (1995)