Finite multiplicity, invariant differential operators and the orbit method Ronald Lipsman multiplicitydifferential operatorsorbit method Indiana University Mathematics Journal 1997 text pdf 10.1512/iumj.1997.46.1194 10.1512/iumj.1997.46.1194 en Indiana Univ. Math. J. 46 (1997) 561 - 574 state-of-the-art mathematics http://iumj.org/access/