1. N. I. Ahiezer, Lectures on the theory of approximation. Second edition, Moscow (1965) [Russian].
2. E. Asplund, Čebyšev sets in Hilbert space. Trans. Amer. Math. Soc. 144 (1969), 236–240.
3. D. A. Ault, F. R. Deutsch, PD. Morris and J. E. Olson, Interpolating subspaces in approximation theory. J. Approx. Theory 3 (1970), 164–182.
4. J. H. Biggs, F. R. Deutsch, R. E. Huff, P. D. Morris and J. E. Olson, Interpolating subspaces in l
1 -spaces (to appear).
5. J. Blatter, Zur Stetigkeit von mengenwertigen metrischen Projektionen. Schriften desRheinisch-Westf. Inst. fur Instrum. Math. Univ. Bonn, Ser.A,.No. 16(1967), 17–38.