1. Birkhoff, G.: J. Math. Mech. 16(1967), 987–990.
2. de Boor, C.: J. Math. Mech. 17(1968), 729–736.
3. de Boor, C.: J. Approx. Thy 6(1972), 50–62.
4. Braess, D.: Numer. Math. 17(1971), 357–366.
5. Burchard, H.: “Splines (with optimal knots) are better”, to appear in J. Applicable Math. 1 (1972).