A Sufficient Condition for Further Reduction of Unknowns in the Einstein-Maxwell Field Equations in the Case of the Characteristic [(11)11] S. Isaak Indiana University Mathematics Journal 1973 text pdf 10.1512/iumj.1973.22.22023 10.1512/iumj.1973.22.22023 en Indiana Univ. Math. J. 22 (1973) 287 - 292 state-of-the-art mathematics http://iumj.org/access/