Principal Curvature and Harmonic Analysis Allan Greenleaf Indiana University Mathematics Journal 1981 text pdf 10.1512/iumj.1981.30.30043 10.1512/iumj.1981.30.30043 en Indiana Univ. Math. J. 30 (1981) 519 - 537 state-of-the-art mathematics http://iumj.org/access/