Edmunds D.E., Triebel H. Function Spaces, Entropy Numbers, Differential Operators

Cambridge: Cambridge University Press, 1996. — 265 p. — (Cambridge Tracts in Mathematics 120).The distribution of the eigenvalues of differential operators has long fascinated mathematicians. Recent advances have shed new light on classical problems in this area, and this book presents a fresh approach, largely based on the results of the authors. The emphasis here is on a topic of central importance in analysis, namely the relationship between i) function spaces on Euclidean n-space and on domains; ii) entropy numbers in quasi-Banach spaces; and iii) the distribution of the eigenvalues of degenerate elliptic (pseudo) differential operators. The treatment is largely self-contained and accessible to nonspecialists.These are the authors and this is their magnum opus
Quite a lot of this stuff has never appeared in a book before