Approximation from shift-invariant subspaces of L 2 ( R d ) L_ 2(mathbf {R}^ d) Academic Article

A complete characterization is given of closed shift-invariant subspaces of L 2 ( R d ) {L_2}({mathbb {R}^d}) which provide a specified approximation order. When such a space is principal (i.e., generated by a single function), then this characterization is in terms of the Fourier transform of the generator. As a special case, we obtain the classical Strang-Fix conditions, but without requiring the generating function to decay at infinity. The approximation order of a general closed shift-invariant space is shown to be already realized by a specifiable principal subspace.