Gallone, MatteoMichelangeli, AlessandroOttolini, Andrea2017-05-312017-05-312017https://openscience.sissa.it/handle/1963/35286The core results of the so-called KreIn-Visik-Birman theory of self-adjoint extensions of semi-bounded symmetric operators are reproduced, both in their original and in a more modern formulation, within a comprehensive discussion that includes missing details, elucidative steps, and intermediate results of independent interest.enSelf-adjoint operatorsself-adjoint extensionsvon Neumann extension theoryKrein-Visik-Birman extension theoryPreprint25/2017/MATE