Composition of Solutions of Linear Partial Differential Equations in Two Independent Variables H. Lewy Indiana University Mathematics Journal 1959 text pdf 10.1512/iumj.1959.8.58013 10.1512/iumj.1959.8.58013 en Indiana Univ. Math. J. 8 (1959) 185 - 192 state-of-the-art mathematics http://iumj.org/access/