@article{casals2017existence,
	title={Existence $h$-principle for Engel structures},
	author={Casals, Roger and P{\'e}rez, Jos{\'e} Luis and del Pino, {\'A}lvaro and Presas, Francisco},
	journal={Inventiones mathematicae},
	volume={210},
	number={2},
	pages={417--451},
	year={2017},
	publisher={Springer}
}