H. Bahouri, J. Chemin, and R. Danchin, Fourier Analysis and Nonlinear Partial Differential Equations, Grundlehren der Mathematischen Wissenschaften (Fundamental Principles of Mathematical Sciences, p.343, 2011.
DOI : 10.1007/978-3-642-16830-7
URL : https://hal.archives-ouvertes.fr/hal-00732127

M. Cicognani and F. Colombini, Modulus of continuity of the coefficients and loss of derivatives in the strictly hyperbolic Cauchy problem, Journal of Differential Equations, vol.221, issue.1, pp.143-157, 2006.
DOI : 10.1016/j.jde.2005.06.019

R. Coifman and Y. Meyer, Au delà des opérateurs pseudo-différentiels, Astérisque, 1978.

F. Colombini, E. De-giorgi, and S. Spagnolo, Sur les équations hyperboliques avec des coefficients qui ne dépendent que du temps, Ann. Scuola Norm. Sup. Pisa Cl. Scienze, issue.4, pp.6-511, 1979.

F. Colombini and D. D. Santo, A Note on Complete Hyperbolic Operators with log-Zygmund Coefficients, J. Math. Sci. Univ. Tokyo, pp.16-95, 2009.
DOI : 10.1007/978-3-319-02550-6_3
URL : https://hal.archives-ouvertes.fr/hal-00795817

F. Colombini, D. Santo, F. Fanelli, and G. Métivier, Time-Dependent Loss of Derivatives for Hyperbolic Operators with Non Regular Coefficients, Communications in Partial Differential Equations, vol.175, issue.10, 2012.
DOI : 10.1080/03605309908820714
URL : https://hal.archives-ouvertes.fr/hal-00733563

F. Colombini, D. Santo, F. Fanelli, and G. Métivier, A well-posedness result for hyperbolic operators with Zygmund coefficients, Journal de Math??matiques Pures et Appliqu??es, vol.100, issue.4
DOI : 10.1016/j.matpur.2013.01.009
URL : https://hal.archives-ouvertes.fr/hal-00863695

F. Colombini and F. Fanelli, A note on non-homogeneous hyperbolic operators with low regularity coefficients, Rend. Istit. Mat. Univ. Trieste, vol.42, pp.1-25, 2010.

F. Colombini and N. Lerner, Hyperbolic operators with non-Lipschitz coefficients, Duke Math, J, vol.77, pp.657-698, 1995.
DOI : 10.1215/s0012-7094-95-07721-7
URL : http://www.numdam.org/article/SEDP_1993-1994____A19_0.pdf

F. Colombini and G. Métivier, The Cauchy problem for wave equations with non Lipschitz coefficients; Application to continuation of solutions of some nonlinear wave equations, Annales scientifiques de l'??cole normale sup??rieure, vol.41, issue.2, pp.41-177, 2008.
DOI : 10.24033/asens.2066

D. and D. Santo, The Cauchy problem for a hyperbolic operator with Log-Zygmund coefficients, Further Progress in Analysis, World Sci. Publ, pp.425-433, 2009.

L. Hörmander, Linear partial differential operators, 1963.

A. E. Hurd and D. H. , Questions of existence and uniqueness for hyperbolic equations with discontinuous coefficients, Transactions of the American Mathematical Society, vol.132, issue.1, pp.159-174, 1968.
DOI : 10.1090/S0002-9947-1968-0222457-8

G. Métivier, Interaction de Deux Chocs Pour un Systeme de Deux Lois de Conservation, en Dimension Deux D'Espace, Transactions of the American Mathematical Society, vol.296, issue.2, pp.431-479, 1986.
DOI : 10.2307/2000375

G. Métivier, Para-differential calculus and applications to the Cauchy problem for nonlinear systems, Centro di Ricerca Matematica " Ennio De Giorgi " (CRM) Series, 5, Edizioni della Normale, 2008.

G. Métivier and K. Zumbrun, Large viscous boundary layers for noncharacteristic nonlinear hyperbolic problems, Memoirs of the American Mathematical Society, vol.175, issue.826, p.175, 2005.
DOI : 10.1090/memo/0826

S. Mizohata, The Theory of Partial Differential Equations, 1973.

S. Tarama, Energy estimate for wave equations with coefficients in some Besov type class, Paper No. 85 (electronic), 2007.