Reflection Geometry for a First Order Elliptic System in Two Independent Variables Oleg CarletonJames Sloss Indiana University Mathematics Journal 1974 text pdf 10.1512/iumj.1974.23.23059 10.1512/iumj.1974.23.23059 en Indiana Univ. Math. J. 23 (1974) 699 - 718 state-of-the-art mathematics http://iumj.org/access/