Approximation orders of shift-invariant subspaces of $W^s_2({\Bbb R}^d)$
  We extend the existing theory of approximation orders provided by shift-invariant subspaces of $L_2$ to the setting of Sobolev spaces, provide treatment of $L_2$ cases that have not been covered before, and apply our results to determine approximation order of solutions to a refinement equation with a higher-dimensional solution space. 
Olga Holtz, Amos Ron