Scalar Invariants and Groups of Motions in a Four Dimensional Einstein Space R. Kerr Indiana University Mathematics Journal 1963 text pdf 10.1512/iumj.1963.12.12003 10.1512/iumj.1963.12.12003 en Indiana Univ. Math. J. 12 (1963) 33 - 54 state-of-the-art mathematics http://iumj.org/access/