1. V.I. Arnol'd, On functions of three variables, Dokl. Akado Nauk SSSR 114 (1957), 679-681. (Russian)
2. A.N. Kolmogorov, On the representation of continuous functions of several variables by the superposition of continuous functions of a smaller number of variables, ibid 108 (1956), 179-182. (Russian)
3. K. Menger, Kurventheorie, Teubner, Leipzig, 1932.
4. A.S. Kronrod, On functions of two variables, Uspehi Mat. Nauk 5, No.1 (35) (1950), 24-133.
5. A.N. Kolmogorov, On the representation of continuous functions of several variables in the form of the superposition of continuous functions of one variable and addition, Dokl. Akad. Nauk SSSR 114 (1957), 953-956. (Russian)