Duality and Perturbations of \Phi_+ and \Phi_-Operators Le Chu Indiana University Mathematics Journal 1977 text pdf 10.1512/iumj.1977.26.26073 10.1512/iumj.1977.26.26073 en Indiana Univ. Math. J. 26 (1977) 905 - 914 state-of-the-art mathematics http://iumj.org/access/