1. Two characterizations of Hilbert spaces by approximation theoretical properties;Bosznay;Ann. Univ. Sci. Budapest Eötvös Sect. Math.,1978
2. Nonexpansive projections on subsets of Banach spaces;Bruck;Pacific J. Math.,1973
3. Normed Linear Spaces;Day,1973
4. Some characterizations of inner-product spaces;Day;Trans. Amer. Math. Soc.,1947
5. Undistorted linear normed spaces;Goryachev;Vestnik Moskov. Univ. Ser. I Mat. Meh.,1970