Classification of Space-Time Curvature Tensor V. Basic Real Characteristics Vaiclav Hlavaty Indiana University Mathematics Journal 1968 text pdf 10.1512/iumj.1968.17.17058 10.1512/iumj.1968.17.17058 en Indiana Univ. Math. J. 17 (1968) 921 - 952 state-of-the-art mathematics http://iumj.org/access/