@inproceedings{peter2010persistence,
title={Persistence of freeness for Lie pseudogroup actions},
author={Peter J. Olver, and Juha Pohjanpelto},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203631166161009},
booktitle={Arkiv for Matematik},
volume={50},
number={1},
pages={165-182},
year={2010},
}
Peter J. Olver, and Juha Pohjanpelto. Persistence of freeness for Lie pseudogroup actions. 2010. Vol. 50. In Arkiv for Matematik. pp.165-182. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203631166161009.