The book is very nice and carefully written, and is a very valuable contribution to the subject . It is well suited to experts but also to Ph.D. students who are willing to pursue this beautiful and rich area of research. (Alberto Parmeggiani, Mathematical Reviews, August, 2019) The present book is devoted to developing general concepts of ellipticity of boundary value problems (BVPs) and provides a self-contained resource for use by professional researchers. (David Kapanadze, zbMATH 1423.35003, 2019)