1. Busemann, H.: Note on a theorem of convex sets. Mat. Tidsskr. B. 1947, 32?34.
2. Day, M. M.: Normed linear spaces. Ergeb. Math. Grenz. no. 21. Berlin 1959.
3. Eflmov, N. V., andS. B. Stechkin: Chebyshev sets in Banach spaces (Russian). Doklady Akad. Nauk SSSR121, 582?585 (1958).
4. Efimov, N. V., andS. B. Stechkin: Some supporting properties of sets in Banach spaces as related to Chebyshev sets (Russian). Doklady Akad. Nauk SSSR127, 254?257 (1959).
5. Fan, Ky, andI. Glicksberg: Some geometric properties of the spheres in a normed linear space. Duke Math. J.25, 553?568 (1958).