Best Approximation and Convergence Properties of Higher-Order Spline Approximations J. AhlbergE. NilsonJ. Walsh Indiana University Mathematics Journal 1965 text pdf 10.1512/iumj.1965.14.14017 10.1512/iumj.1965.14.14017 en Indiana Univ. Math. J. 14 (1965) 231 - 243 state-of-the-art mathematics http://iumj.org/access/