Function Spaces, Entropy Numbers, Differential OperatorsThe 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;