AMS-Code: Primary 22E45, 22E70, 43A32, 44A20, 46L60, 47B49; Secondary 81R05, 81R30, 81T05. Dedicated to the memories of Irving E. Segal and Harish-Chandra. Appears in The Mathematical Legacy of Harish-Chandra: A Celebration of Representation Theory and Harmonic Analysis (R. Doran and V. Varadarajan, eds.), Proc. Sympos. Pure Math., vol. 68, American Mathematical Society, Providence, R.I., 2000, pp. 333--401.