Fredholm Criteria for $G$-pseudodifferential Operators

Let $G$ be a compact Lie group that acts smoothly on a closed manifold $M$. Using a general Simonenko principle, we derive a novel criterion for the Fredholm property of $G$-pseudodifferential operators acting on Sobolev spaces of sections of vector bundles over $M$. In case the group is finite, we obtain a further characterization of the Fredholm property of $G$-pseudodifferential operators in terms of the invertibility of suitable symbols.