Singularity-Free Regions for Solutions of 2nd Order Non-Linear Differential Equations K. Das Indiana University Mathematics Journal 1964 text pdf 10.1512/iumj.1964.13.13004 10.1512/iumj.1964.13.13004 en Indiana Univ. Math. J. 13 (1964) 73 - 84 state-of-the-art mathematics http://iumj.org/access/