@inproceedings{g.2000smoothness,
title={Smoothness of Lipschitz-continuous graphs with nonvanishing Levi curvature},
author={G. Citti, E. Lanconelli, and A. Montanari},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203339903723621},
booktitle={Acta Mathematica},
volume={188},
number={1},
pages={87-128},
year={2000},
}
G. Citti, E. Lanconelli, and A. Montanari. Smoothness of Lipschitz-continuous graphs with nonvanishing Levi curvature. 2000. Vol. 188. In Acta Mathematica. pp.87-128. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203339903723621.