On a comparison between absolute and relative self-adjoint extension schemes

The problem of connecting the operator parameters that label the same self-adjoint extension of a given symmetric operator, respectively, within the ‘absolute’ von Neumann extension scheme and the ‘relative’ boundary-triplet-induced extension scheme (i.e., a la Kreı̆n-Višik-Birman) is discussed, and quantitative con- nections between the two parameters are established in the limit of deficiency spaces at complex spectral points converging to the deficiency space at a real spectral point.