Erdös, P., Szabados, J., Varma, A. K., and Vértesi, P., On an interpolation theoretical extremal problem. Studia Sci. Math. Hungar. 29 (1994), no. 1-2, 55--60.
DeVore, R. and Szabados, J., Saturation theorems for discretized linear operators. Anal. Math. 1 (1975), no. 2, 81--89.
DeVore, Ronald A. and Scott, L. Ridgway, Error bounds for Gaussian quadrature and weighted-L1 polynomial approximation. SIAM J. Numer. Anal. 21 (1984), no. 2, 400--412.
Arnold, Douglas N., Scott, L. Ridgway, and Vogelius, Michael S., Regular inversion of the divergence operator with Dirichlet boundary conditions on a polygon. Ann. Scuola Norm. Sup. Pisa Cl. Sci. (4) 15 (1988), no. 2, 169--192.
Here is another path (there are many):
Erdös, P. and Kormornik, V., Developments in non-integer bases. Acta Math. Hungar. 79 (1998), no. 1-2, 57-83.
Baiocchi, C., Kormornik, V., and Loreti, P., Théories du type Ingham et application à la théorie du contrôle. C. R. Acad. Sci. Paris Sci. I Math. 326 (1998) no. 4, 453-458.
Baiocchi, C. and Brezzi, F., Optimal error estimates for linear parabolic problems under minimal regularity assumptions. Calcolo 20 (1983), no. 2, 143-176.
Arnold, D. and Brezzi, F., Mixed and nonconforming finite element methods: implementation, postprocessing and error estimates. RAIRO Mod. Math. Anal. Num. 19 (1985), no. 1, 7-32.