1. Evolution of perturbations on a weakly inhomogeneous background;Kulikovskii;J Appl Math Mech,2007
2. Operational Methods;Maslov,1976
3. Linear and Nonliniear Waves;Whitham,1974
4. Geometrical Optics of Inhomogeneous Media;Kravtsov YuA,2011
5. Physical Kinetics;Lifshitz,1980