1. Birkhoff, G.: Orthogonality in linear metric spaces. Duke Math. J. 1, 169?172 (1935).
2. Bruck, Jr., R. E.: Nonexpansive projections on subsets of Banach spaces. Pacific J. Math. 47, 341?355 (1973).
3. Cheney, E. W., andK. H. Price: Minimal projections. In: Approximation Theory (Ed.A. Talbot), p. 261?289. New York: Academic Press. 1970.
4. Day, M. M.: Review of the paper ofJ. R. Holub ?Rotundity, orthogonality and characterizations of inner product spaces?. Math. Rev. 52, 175 (1976) ? 1263.
5. Franchetti, C., andM. Furi: Some characteristic properties of real Hilbert spaces. Rev. Roumaine Math. Pures Appl. 17, 1045?1048 (1972).