Radial basis function interpolation has attracted a lot of interest in recent years. For popular choices, for example thin plate splines, this problem has a variational formulation, i.e. the interpolant minimizes a semi-norm on a certain space of radial functions. This gives rise to a function space, called the native space. Every function in this space has the property that the semi-norm of an arbitrary interpolant to this function is uniformly bounded. In applications it is of interest whether a sufficiently smooth function belongs to the native space. In this paper we give sufficient conditions on the differentiability of a function with compact support, in the case of cubic, linear and thin plate splines. In the case of multiquadrics and Gaussian functions, it is shown that the only compactly supported function that satisfies these conditions is identically zero. 2001 Academic Press * i ,(&x&x i &)+ p(x), x # R d . (1.1) In this paper we consider the following choices of ,: ,(r)=r (linear), ,(r)=r 3 (cubic), ,(r)=r 2 log r (thin plate spline), = r 0. (1.2)
,(r)=-r 2 +c 2 (multiquadric), ,(r)=e &r 2 (Gaussian),