Julia Operators and Complementation in Krein Spaces Michael DritschelJames Rovnyak Indiana University Mathematics Journal 1991 text pdf 10.1512/iumj.1991.40.40039 10.1512/iumj.1991.40.40039 en Indiana Univ. Math. J. 40 (1991) 885 - 901 state-of-the-art mathematics http://iumj.org/access/