Wiener-Hopf Difference Equations Gilbert Strang Indiana University Mathematics Journal 1964 text pdf 10.1512/iumj.1964.13.13005 10.1512/iumj.1964.13.13005 en Indiana Univ. Math. J. 13 (1964) 85 - 96 state-of-the-art mathematics http://iumj.org/access/