Discrete Potential Theory and Toeplitz Operators on the Quarter-Plane, I Stanley Osher Indiana University Mathematics Journal 1975 text pdf 10.1512/iumj.1975.24.24071 10.1512/iumj.1975.24.24071 en Indiana Univ. Math. J. 24 (1975) 887 - 896 state-of-the-art mathematics http://iumj.org/access/