[1] 
G. W. Zumbusch.
Simultanous hp Adaptation in Multilevel Finite Elements.
ISBN 3826511360. Shaker, Aachen, Germany, 1996. [ bib  .ps.gz 1  .pdf 1  .ps.gz 1 ] An important tool in engineering is the finite element method.... The combination of both methods, the hp version, supplies the preasymptotic exponentially convergent pversion continuously with properly adapted grids. Hence it achieves the superior exponential convergence asymptotically, too, instead of algebraic convergence of its ingredients the hversion and the pversion. Although the first theoretical results claiming these convergence rates are quite classic, the number of codes using the hpversion of finite elements is still rather limited. Reasons for that are the pure implementational complexity and the details, in conjunction with the rumor of engineers' low precision requirements. But the major reason is the lack of a robust (self) adaptive control delivering the desired exponential convergence. ...
