Referat na podstawie prac:

1. Ronald A. DeVore and Vasil A. Popov, Free Multivariate Splines, Constr. Approx. 3 (1987), 239-248.
2. A. Cohen, R. DeVore, P. Petrushev, H. Xu, Nonlinear approximation and the space $BV(R^2)$, Amer. J. Math. 121 (1999), 587-628.