1. : Philosophie der Mathematik und Naturwissenschaft, Oldenbourg, München. 1966, p. 352.
2. See, e.g.: (a) : Group Theory and its Application to Physical Problems. Addison-Wesley, Reading, Mass. 1962;
3. (b) : Symmetry. Pergamon, New York 1963;
4. (c) : Set Theory. McGraw-Hill, New York 1964;
5. (d) : General Topology. McGraw-Hill, New York 1965;