@article{ainsworthkay1999,
  title = "The approximation theory for the p-version finite element method and application to non-linear elliptic PDE's",
  author = "M. Ainsworth and D. Kay",
  year = "1999",
  journal = "Num. Math.",
  pages = "351--388",
  volume = "82",
  doi = "10.1007/s002110050423",
}