1. Geometrical Optics of Non-uniform Media;Kravtsov,1980
2. Wave Propagation in Anisotropic Elastic Media;Petrashen',1980
3. Ray Methods in the Theory of Wave Propagation and Scattering;Podil'chuk,1988
4. The Characteristics of Differentiable Mappings;Arnol'd,1982
5. The critical points of functions and the classification of caustics;Arnol'd;Uspekhi Mat. Nauk,1974