Wan-Chiu Li Bruno Vallet Nicolas Ray Bruno Lévy Representing Higher-Order Singularities in Vector Fields on Piecewise Linear Surfaces. 1315-1322 2006 12 IEEE Trans. Vis. Comput. Graph. 5 http://doi.ieeecomputersociety.org/10.1109/TVCG.2006.173 db/journals/tvcg/tvcg12.html#LiVRL06