1 “... of characteristic surfaces and use it to classify partial differential equations. The discussion grows out...”

2 “...This self-contained textbook offers an elementary introduction to partial differential equations...”

3 “..., and hints toward solutions, making this a timely resource for students. Partial Differential Equations...”

4 “... of the notions of degeneracy and singularity in partial differential equations.  Although related in spirit to a...”

5