1. Espaces Vectoriels Topologiques;Bourbaki,1966
2. Vererbbarkeitsaussagen für abgeschlossene algebraische Komplementärräume in topologischen Vektorräumen;Dierolf;Archiv der Mathematik,1976
3. Perturbation theory and strictly singular operators in locally convex spaces;Dulst;Studia Math,1970
4. On strictly singular and strictly cosingular operators. I and II;Pełczyński;Bull. de l'Acad. Pol. des Science,1965
5. On strictly cosingular operators in locally convex spaces;Vladimirskiǐ;Studia Math,1970