Hans Sagan: 2 books

This well-known text uses a limited number of basic concepts and techniques — Hamilton's principle, the theory of the first variation and Bernoulli's separation method — to develop complete solutions to linear boundary value problems associated with second order partial differential equations...

". . . eminently suitable as a text for an introductory course: the style is pleasant; the prerequisites are kept to a minimum . . . and the pace of the development is appropriate for most students at the senior or first year graduate level." — American Mathematical MonthlyThe purpose of this text...