@inproceedings{piotr2001approximation,
title={Approximation in Sobolev spaces of nonlinear expressions involving the gradient},
author={Piotr Hajłasz, and Jan Malý},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203604758338793},
booktitle={Arkiv for Matematik},
volume={40},
number={2},
pages={245-274},
year={2001},
}

Piotr Hajłasz, and Jan Malý. Approximation in Sobolev spaces of nonlinear expressions involving the gradient. 2001. Vol. 40. In Arkiv for Matematik. pp.245-274. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203604758338793.