1. American Mathematical Society Colloquium Publications, Vol. 31;Hille, Einar,1948
2. C. Kuratowski, Topologie I, Monografie Matematyczne III, Warsaw-Lwów, 1933.
3. Images of sets satisfying the conditions of Baire;McShane, E. J.;Ann. of Math. (2),1950
4. On continuity and openness of homomorphisms in topological groups;Pettis, B. J.;Ann. of Math. (2),1950