1. Schoenberg I. J., “On metric arcs of vanishing Menger curvature,” Ann. of Math. (2), 41, No. 4, 715–726 (1940).
2. Schoenberg I. J., “A remark on M. M. Day’s characterization of inner-product spaces and a conjecture of L. M. Blumenthal,” Proc. Amer. Soc., 3, No. 6, 961–964 (1952).
3. Berger M., Geometry. Vol. 1 [Russian translation], Mir, Moscow (1984).
4. Blumenthal L. M., Theory and Applications of Distance Geometry, Clarendon Press, Oxford (1953).
5. Menger K., “Untersuchunger über allgemeine Metrik. I–III,” Math. Ann., Bd 100, 75–163 (1928).