@article{ainsworth1999approximation,
  title = "The approximation theory for the p-version finite element method and application to non-linear elliptic PDEs",
  author = "Ainsworth, Mark and Kay, David",
  year = "1999",
  journal = "Numerische Mathematik",
  number = "3",
  pages = "351--388",
  publisher = "Springer Berlin/Heidelberg",
  volume = "82",
}