The notion of quasi boundary triples and their Weyl functions is an abstract
concept to treat spectral and boundary value problems for elliptic partial
differential equations. In the present paper the abstract notion is further
developed, and general theorems on resolvent differences belonging to operator
ideals are proved. The results are applied to second order elliptic
differential operators on bounded and exterior domains, and to partial
differential operators with δ and δ′-potentials supported on
hypersurfaces.Comment: 40 pages, submitte