Stephan Dahlke Erich Novak Winfried Sickel Optimal approximation of elliptic problems by linear and nonlinear mappings I. 29-49 2006 22 J. Complexity 1 http://dx.doi.org/10.1016/j.jco.2005.06.005 db/journals/jc/jc22.html#DahlkeNS06