Symmetric Einstein Spaces and Spectral Geometry Harold Donnelly Indiana University Mathematics Journal 1975 text pdf 10.1512/iumj.1975.24.24045 10.1512/iumj.1975.24.24045 en Indiana Univ. Math. J. 24 (1975) 603 - 606 state-of-the-art mathematics http://iumj.org/access/