“…To set the stage, we also verify Krein-type resolvent formulas which relate the inverses (A − zI L 2 ( ) ) −1 and A * − zI L 2 ( ) −1 with the resolvents of the Dirichlet realizations A D and A D , respectively. For the self-adjoint case, such formulas are well-known and can be found, for example, in [1,8,9,11,15,29,30,48,58,59]. For dual pairs of elliptic differential operators we refer to [14], and for a more abstract operator theory framework, see [49] and [50].…”