Frequency Domain Design and Analytic Selections J. William HeltonDonald Marshall Indiana University Mathematics Journal 1990 text pdf 10.1512/iumj.1990.39.39010 10.1512/iumj.1990.39.39010 en Indiana Univ. Math. J. 39 (1990) 157 - 184 state-of-the-art mathematics http://iumj.org/access/