@TechReport{ Griebel.Knapek:1999, author = {M.~Griebel and S.~Knapek}, title = {Optimized approximation spaces for operator equations}, institution = {SFB 256, Univ. Bonn}, year = {1999}, number = {568}, pdf = {http://wissrech.ins.uni-bonn.de/research/pub/knapek/duenn6.pdf} , annote = {unrefereed,245C,report} }