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Hassi Seppo and de Snoo H. S. V. and Szafranieć Franciszek Hugon: Normal intermediate extensions of symmetric relations. In: Acta scientiarum mathematicarum, (80) 1-2. pp. 195-232. (2014)
Fleige Andreas and Hassi Seppo and Snoo Hendrik Simon Victor de: A Kreĭn space approach to representation theorems and generalized Friedrichs extensions. In: Acta scientiarum mathematicarum, (66) 3-4. pp. 633-650. (2000)
Hassi Seppo and Kaltenbäck Michael and Snoo Hendrik Simon Victor de: Generalized Kreĭn-von Neumann extensions and associated operator models. In: Acta scientiarum mathematicarum, (64) 3-4. pp. 627-655. (1998)