  For a second order formally symmetric elliptic differential expression we
show that the knowledge of the Dirichlet-to-Neumann map or Robin-to-Dirichlet
map for suitably many energies on an arbitrarily small open subset of the
boundary determines the self-adjoint operator with a Dirichlet boundary
condition or with a (possibly non-self-adjoint) Robin boundary condition
uniquely up to unitary equivalence. These results hold for general Lipschitz
domains, which can be unbounded and may have a non-compact boundary, and under
weak regularity assumptions on the coefficients of the differential expression.
