Approximation order from bivariate C1-cubics on a four-directional mesh is full

@article{Lai1994ApproximationOF,
title={Approximation order from bivariate C1-cubics on a four-directional mesh is full},
author={Ming-Jun Lai},
journal={Computer Aided Geometric Design},
year={1994},
volume={11},
pages={215-223}
}

We show that the space of bivariate C 1 piecewise cubic polynomial functions on a four directional mesh of size h has the full approximation order, i.e., O(h 4).