1. J. Favard, “Sur l’interpolation,” J. Math. Pures Appl. 19 9, 281–306 (1940).
2. S. Fisher and J. Jerome, Minimum Norm Extremals in Function Spaces (Springer, Berlin, 1975), Ser. Lecture Notes in Mathematics, Vol. 479.
3. V. M. Tikhomirov and B. D. Boyanov, “Some convex problems of approximation theory,” Serdica 5 1, 83–96 (1979).
4. Yu. N. Subbotin, “On the relations between finite differences and the corresponding derivatives,” Proc. Steklov Inst. Math. 78, 23–42 (1965).
5. Yu. N. Subbotin, “Functional interpolation in the mean with smallest nth derivative,” Trudy Mat. Inst. Steklova 88, 30–60 (1967).