Published online by Cambridge University Press: 22 June 2005
The main aim of this paper is to give an overview of work that started in the early 90's and contained non-linearly in a number of papers and preprints. The style will be informal since the author wants this paper to be accessible to non-experts. Formal definitions, such as “$\Omega$ is a Lipschitz domain” will not be given. They will be well known to experts and “intuitively” clear to non-experts. Technical statements or proofs will not be given either. The reader who is not happy with this may consult the references given. The reader who wants to read this overview and meets unknown words is advised in the first instance to ignore them.