Branching of Singularities for a Class of Hyperbolic Operators Serge Alinhac Indiana University Mathematics Journal 1978 text pdf 10.1512/iumj.1978.27.27071 10.1512/iumj.1978.27.27071 en Indiana Univ. Math. J. 27 (1978) 1027 - 1037 state-of-the-art mathematics http://iumj.org/access/