1. N. Bourbaki, Éléments de Mathématique, Livre III, Topologie Generale, Chapters I, II, Paris, 1940.
2. \bysame, Éléments de Mathématique, Livre III, Topologie Generale, Chapter IX, Paris, 1948.
3. S. P. Diliberto, Pertubation of continuous surfaces, II, Technical Report No. 9, O.N.R. Research Contract Nonr-222(37), Dept. of Mathematics, University of California at Berkeley, 1957.
4. N. Dunford and J. T. Schwartz, Linear operators, vol. 1, Chapter 5, to appear.
5. American Mathematical Society Colloquium Publications, Vol. 36;Gottschalk, Walter Helbig,1955