On the Invertibility of a Class of Toeplitz Operators on the Quarter-Plane R. Douglas Indiana University Mathematics Journal 1972 text pdf 10.1512/iumj.1972.21.21083 10.1512/iumj.1972.21.21083 en Indiana Univ. Math. J. 21 (1972) 1031 - 1035 state-of-the-art mathematics http://iumj.org/access/