1. Theory of Approximation;Achieser,1956
2. Piecewise polynomial approximation on optimal meshes
3. Dahmen, W. and K. Scherer, Best approximation by piece-wise polynomials with variable knots and degrees, preprint.
4. Rice, J., On the degree of convergence of nonlinear spline approximation, Proc. Symp. on Approx. Theory, Madison, I. J. Schoenberg (Ed.), Academic Press., 1969, 349–365.