The Structure of a Class of Huygens' Operators* J. Lagnese Indiana University Mathematics Journal 1969 text pdf 10.1512/iumj.1969.18.18086 10.1512/iumj.1969.18.18086 en Indiana Univ. Math. J. 18 (1969) 1195 - 1201 state-of-the-art mathematics http://iumj.org/access/