Contributions to the extension theory in Hilbert spaces

This thesis presents solutions to certain problems in the extension theory in Hilbert spaces. Basically there are three main parts of the thesis corresponding to three objects which are studied. The first part consists of chapter 3 and deals with Lagrangian pairs in a pair of Hilbert spaces. In the second part nonnegative linear relations are studied. Finally, in the third part certain results concerning the extension theory of sectorial relations in Hilbert spaces are presented