Mathematics of Computation, Vol. 34, No. 150 (Apr., 1980), pp. 441-463 (23 pages) Constructive proofs and several generalizations of approximation results of J. H. Bramble and S. R. Hilbert are ...

A characterization is presented of the class of stationary processes that have polynomial covariance functions of degree less than or equal to 4 on an interval. The results extend to isotropic random ...