The Development of Singularities of Solutions of Nonlinear Hyperbolic Equations of Order Greater Than Unity A. Jeffrey Indiana University Mathematics Journal 1966 text pdf 10.1512/iumj.1966.15.15038 10.1512/iumj.1966.15.15038 en Indiana Univ. Math. J. 15 (1966) 585 - 598 state-of-the-art mathematics http://iumj.org/access/