1. Christensen, J.P.R.: Topology and Borel structure. Notas de Matematica. Amsterdam: North-Holland 1974
2. Klee, V., Wilansky, A.: Research problems no. 13. Bull. Amer. Math. Soc.72, 656?656 (1966)
3. Lacey, E.: Separable quotients of Banach spaces. An. Acad. Brasil. Ci.44, 185?189 (1972)
4. Munroe, M.E.: Measure and integration, 2. ed. Reading: Addison-Wesley 1971
5. Rosenthal, H.P.: Quasicomplemented subspaces of Banach spaces. J. Functional Analysis4, 176?214 (1969)