Chan, T.; Zhu, W. (2005). Level set based shape prior segmentation. IEEE Computer Society Conference on Computer Vision and Pattern Recognition. doi:10.1109/CVPR.2005.212.
Gilbarg & Trudinger 1983, Lemma 14.16. Gilbarg, D.; Trudinger, N. S. (1983). Elliptic Partial Differential Equations of Second Order. Grundlehren der mathematischen Wissenschaften. Vol. 224 (2nd ed.). Springer-Verlag. doi:10.1007/978-3-642-61798-0. (or the Appendix of the 1977 1st ed.)
Gilbarg & Trudinger 1983, Equation (14.98). Gilbarg, D.; Trudinger, N. S. (1983). Elliptic Partial Differential Equations of Second Order. Grundlehren der mathematischen Wissenschaften. Vol. 224 (2nd ed.). Springer-Verlag. doi:10.1007/978-3-642-61798-0. (or the Appendix of the 1977 1st ed.)