@article{kravtsov:1968:tna,
    author = {Kravtsov, Y. A.},
     title = {Two new asymptotic methods in the theory of wave propagation
              in inhomogeneous media},
   journal = {Sov. Phys. Acoust.},
    volume = {14},
      year = {1968},
     pages = {1–17},
      issn = {0038-562X}}