Classification of Space-Time Curvature Tensor VIa. Complex Basic Characteristics of the Types I, II, IV Vaiclav Hlavaty Indiana University Mathematics Journal 1969 text pdf 10.1512/iumj.1969.18.18023 10.1512/iumj.1969.18.18023 en Indiana Univ. Math. J. 18 (1969) 237 - 281 state-of-the-art mathematics http://iumj.org/access/