Geometric Invariance of Singular Solutions of Some Nonlinear Partial Differential Equations Laurent Veron Indiana University Mathematics Journal 1989 text pdf 10.1512/iumj.1989.38.38003 10.1512/iumj.1989.38.38003 en Indiana Univ. Math. J. 38 (1989) 75 - 100 state-of-the-art mathematics http://iumj.org/access/