Brasche J. F.
Ukr. Mat. Zh. - 1994. - 46, № 1-2. - pp. 37–54
The extension problem for closed symmetric operators with a gap is studied. A new kind of parametrization of extensions (the so-called Krein model) is developed. The notion of a singular operator plays the key role in our approach. We give the explicit description of extensions and establish the spectral properties of extended operators.