In the study of short-wave diffraction problems, asymptotic methods -
the ray method, the parabolic equation method, and its further
development as the "etalon" (model) problem method - play an important
role. These are the meth- ods to be treated in this book. The
applications of asymptotic methods in the theory of wave phenomena are
still far from being exhausted, and we hope that the techniques set
forth here will help in solving a number of problems of interest in
acoustics, geophysics, the physics of electromagnetic waves, and perhaps
in quantum mechanics. In addition, the book may be of use to the
mathematician interested in contemporary problems of mathematical
physics. Each chapter has been annotated. These notes give a brief
history of the problem and cite references dealing with the content of
that particular chapter. The main text mentions only those pUblications
that explain a given argument or a specific calculation. In an effort to
save work for the reader who is interested in only some of the problems
considered in this book, we have included a flow chart indicating the
interdependence of chapters and sections.