Elliptic Partial Differential Operators And Symplectic Algebra

Describes the set of all self-adjoint operators (not just those defined by differential boundary conditions) which are generated by a linear elliptic partial differential expression $A(\mathbf{x},D)=\sum_{0\,\leq\,\lefts\right\,\leq\,2m}a_{s} (\mathbf{x})D^{s}\;\text{for all}\;\mathbf{x}\in\Omega$ in a region $\Omega$.