Theorem: Assume that X is either spherical or affine. Then the center Z(X) of the ring of G-invariant differential operators on X is a polynomial ring. More precisely, Z(X) is isomorphic to the ring of invariants of a finite reflection group.
Appeared in: Annals of Mathematics, Series II 140 (1994) 253-288
Available files:
DVI-file (needs the symbol font lams3)
Postscript file
Pdf-file
Original article (Jstor)