Uniqueness Theorems for the Tricomi Problem. II M. Protter Indiana University Mathematics Journal 1955 text pdf 10.1512/iumj.1955.4.54027 10.1512/iumj.1955.4.54027 en Indiana Univ. Math. J. 4 (1955) 721 - 732 state-of-the-art mathematics http://iumj.org/access/