The approximation theory for the $p$-version finite element method and application to non-linear elliptic PDEs. Ainsworth, M. and Kay, D. Numerische Mathematik, 82(3):351--388, Springer, 1999. bibtex @article{ ainsworth1999approximation,
title = {The approximation theory for the $p$-version finite element method and application to non-linear elliptic {PDE}s},
author = {Ainsworth, M. and Kay, D.},
journal = {Numerische Mathematik},
volume = {82},
number = {3},
pages = {351--388},
year = {1999},
publisher = {Springer}
}