The Holonomy Group VI. Weyl Space W_4, Second Part Vaiclav Hlavaty Indiana University Mathematics Journal 1962 text pdf 10.1512/iumj.1962.11.11003 10.1512/iumj.1962.11.11003 en Indiana Univ. Math. J. 11 (1962) 35 - 59 state-of-the-art mathematics http://iumj.org/access/