Boundary value problems for elliptic partial differential operators on bounded domains

For a symmetric operator or relation A with infinite deficiency indices in a Hilbert space we develop an abstract framework for the description of symmetric and self-adjoint extensions AΘ of A as restrictions of an operator or relation T which is a core of the adjoint A∗. This concept is applied to second order elliptic partial differential operators on smooth bounded domains, and a class of elliptic problems with eigenvalue dependent boundary conditions is investigated.