The first edition of this book gave a systematic exposition of the Weinstein method of calculating lower bounds of eigenvalues by means of intermediate problems. This second edition presents new developments in the framework of the material contained in the first edition, which is retained in somewhat modified form.

S.H. Gould was a professor of mathematics. He held academic posts at the University of Wisconsin, Purdue University, the University of Kansas, and Williams College. He was also the Executive Editor of Mathematical Reviews.