I am studying an article http://arxiv.org/abs/quant-ph/9907069 and having some problems understanding it. Is self adjointness of an operator a sufficient or necessary and sufficient requirement for its eigen vectors with the generalized eigenvectors (i dont know what are these) to form complete set? I want to read the proof too. The references include German texts which are not accessible for me right now. P.S I have tried googling but couldnt find the answer.