1. P.S. Alexandroff and V. Niemytzki, The condition of metrizability of topological spaces and the axiom of symmetry, Mat. Sb. 3 (45) 663–672 (in Russian).
2. Une condition nécessaire et suffisante pour qu'une class (L) soit une class (D);Alexandroff;C. R. Acad. Sci. Paris,1923
3. Mappings and spaces;Arhangel'skii;Russian Math. Surveys,1966
4. Metrization of topological spaces;Bing;Canad. J. Math.,1951
5. Topological Spaces;Čech,1966