Metric Null Parameter and Conformal Null Parameter of a Minimal Geodesic Vaiclav Hlavaty Indiana University Mathematics Journal 1963 text pdf 10.1512/iumj.1963.12.12042 10.1512/iumj.1963.12.12042 en Indiana Univ. Math. J. 12 (1963) 643 - 653 state-of-the-art mathematics http://iumj.org/access/