MSC-Code: 35P10 47A55 47B44 81Q15 (2000 Mathematics Subject Classification) 
PACS-Code: 03.65.Yz 03.65.Db (2001 Physics and Astronomy Classification Scheme) 
 
Weierstrass Institute for Applied Analysis and Stochastics Preprint No. 650,  
Berlin 2001, http://www.wias-berlin.de/publications/preprints/650