# Actions of finite groups on exotic smooth manifolds of dimension >4

Let $M_1^n$ and $M_2^n$, $n>4$ be two smooth compact manifolds that are homeomorphic but not diffeomorphic. Suppose that a finite group is $G$ acting faithfully on $M_1^n$ by diffeomorphisms. Is it true that $G$ admits as well a faithful action on $M_2^n$ by diffeomorphisms?

If no, what would be the a (relatively) simple example?

For example, can one differentiate exotic structures on $S^7$ this way?

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there is absolutely no reason to expect for something like this to be true. Finding explicit counterexamples is quite a different story. I don't know of any examples in dimensions above 4 if you don't restrict the action in any way. For example I believe it's not known if every exotic sphere admits a circle action. –  Vitali Kapovitch May 9 '12 at 20:05
@aglearner: Reinhard Schultz had a sequence of papers titled "Differentiable group actions on homotopy spheres" in 1980s, you should check them for such examples. –  Misha May 9 '12 at 20:45
you might also look at Weinberger's articles on propagating group actions, e.g. ams.org/mathscinet-getitem?mr=910951 –  Paul May 10 '12 at 1:41

The OP doesn't link the actions of $G$ on $M_1$ and $M_2$ in any way. so the action of $Z_2$ on an exotic sphere is not required to be orientation reversing. –  Vitali Kapovitch May 9 '12 at 20:07