Krein resolvent formulas for elliptic boundary problems in nonsmooth   domains
  The paper reports on a recent construction of M-functions and Krein resolvent formulas for general closed extensions of an adjoint pair, and their implementation to boundary value problems for second-order strongly elliptic operators on smooth domains. The results are then extended to domains with $C^{1,1}$ H\"older smoothness, by use of a recently developed calculus of pseudodifferential boundary operators with nonsmooth symbols. 
Gerd Grubb