Key points are not available for this paper at this time.
Duality transformations, i. e. , rotations of electric and magnetic fields into each other, are implementable by a time-local generator for source-free Maxwell theory in an arbitrary spacetime metric. The Maxwell action (E^2-B^2) as well as the stress tensor components, e. g. , E^2+B^2, are duality invariant and the generator is conserved. In the formally analogous Yang-Mills case, duality transformations cannot even be consistently implemented.
Deser et al. (Mon,) studied this question.