@article{garcia_sotomayor_2002,  title={Umbilic and Tangential Singularities on Configurations of Principal Curvature Lines},  volume={74},  ISSN={0001-3765},  url={https://doi.org/10.1590/S0001-37652002000100001},  DOI={10.1590/S0001-37652002000100001},  number={1},  journal={Anais da Academia Brasileira de Ciências},  publisher={Academia Brasileira de Ciências},  author={GARCIA, RONALDO and SOTOMAYOR, JORGE},  year={2002},  month={Mar},  pages={1–17} }