1. R. Courant, Partial Differential Equations [Russian translation], Mir, Moscow (1964).
2. V. G. Gogoladze, “Wave equation for nonhomogeneous and anisotropic media,” Tr. Mat. Inst. Steklova, 9, 107–166 (1935).
3. V. M. Babich and A. A. Klimova, “Hyperbolic equation with a large parameter by lower-order terms and hierarchy of waves,” Algebra Anal., 6, No. 5, 126–171 (1994).
4. A. V. Borovskikh, “Two-dimensional eikonal equation,” Sib. Mat. Zh., 47, No. 5, 993–1018 (2006).
5. A. V. Borovskikh, “Eikonal equation in nonhomogeneous media,” Dokl. Ross. Akad. Nauk, 391, No. 5, 587–590 (2003).