If
k is a field and
R is a commutative
k-algebra, we explore the question of when the ring
of
k-linear differential operators on
R is isomorphic to its opposite ring. Under mild hypotheses, we prove this is the case whenever
R Gorenstein local or when
R is a ring of invariants. As a key step in the proof we show that in many cases of interest canonical modules admit right
... [Show full abstract] D-module structures. After this work was completed we realized that some of our results were already proved in higher generality by Yekutieli, albeit using more sophisticated methods.