 The approximation error is measured in the norm of the isotropic Sobolev space W^{1,p \mathbb I%DIFDELCMD < }%%% ^d).
<meaning-changed>  The approximation error is measured in the norm of the isotropic Sobolev space W^{1,p \mathbb I%DIFDELCMD < }%%% ^d).
 The approximation error is measured in the norm of the isotropic Sobolev space W^{1,p %DIFDELCMD < }%%% ^d).