1. N. V. Efimov and S. B. Stechkin, “Some properties of Chebyshev sets,” Dokl. Akad. Nauk SSSR 118(1), 17–19 (1958).
2. E. S. Polovinkin and M. V. Balashov, Elements of Convex and Strongly Convex Analysis (Fizmatlit, Moscow, 2007) [in Russian].
3. A.V. Arutyunov, “Convex properties of the Legendre transform,” Mat. Zametki 28(2), 255–264 (1980) [Math. Notes 28 (2), 591–595 (1980)].
4. L. P. Vlasov, “Approximative properties of sets in normed linear spaces,” Uspekhi Mat. Nauk 28(6), 3–66 (1973) [Russian Math. Surveys 28 (6), 1–66 (1973)].
5. E. Asplund and B. Grünbaum, “On the geometry of Minkowski planes,” Enseignement Math. (2) 6, 299–306 (1960).