Riemannian manifolds whose skewd-symmetric curvature operator has constant eigenvalues Peter GilkeyJohn LeahyHal Sadofsky Indiana University Mathematics Journal 1999 text pdf 10.1512/iumj.1999.48.1699 10.1512/iumj.1999.48.1699 en Indiana Univ. Math. J. 48 (1999) 615 - 634 state-of-the-art mathematics http://iumj.org/access/