@article{Du2005APF,
  title={A phase field formulation of the Willmore problem},
  author={Qiang Du and Chun Liu and Rolf Ryham and Xiaoqiang Wang},
  doi = {10.1088/0951-7715/18/3/016},
  journal={Nonlinearity},
  year={2005},
  volume={18},
  pages={1249 - 1267}
}