I am interested to know how flexible are Riemannian metrics on a (smooth) manifold $M$.

Let $f:M \rightarrow M$ be a diffeomorphism which admits some Riemannain metric on $M$ making it an isometry. How many different metrics are preserved by $f$? What can we say about $Met(f)=\{g|f^*g=g\}$? (besides that it is a convex cone which is closed under pullback by $f$).

Clearly, it depends on who $f$ is. If $f=Id$ then any Riemannian metric is preserved. 

**Question:**

Assuming $f \neq Id$, is it true that $Met(f)$ is always a finite dimensional manifold? (It is known that the set of all Riemannian metrics is an **infinite** dimensional manifold, I am trying to see how restrictive is the requirment to be preserved by a diff')

What happens if we assume $f$ has no fixed points? (or has only "a small amount" of them in some suitable sense such as $Fix(F)$ is contained in a lower dimensional submanifold of $M$)


**Partial Results and further Questions:**

(1) For the case of $M=\mathbb{R^n}$ and $f\in GL(\mathbb{R^n})$ , since we can assume $f \in O(n)$ (w.r.t to a suitable basis, [see here][1]), we can obtain results based on the particular form of $f$. For instance, the inner product on subspaces where the restriction of $f$ is a proper rotation, is uniquely determined (up to scalar multiple). ([See analysis and sumary here][2]).


(2) Fix some Riemannian metric $g$ on $M$.
A natural point of interest is the correspondence (something analogous to Galois Correspondence) between subgroups $K \subseteq Iso(M,g)$ and $Met(K)= \{h|f^*h=h , \forall f \in K\}$. Of course this is of interest only when $Iso(M,g)$ is "rich" (if $Iso(M,g)= {Id}$ for instance this is clearly uninteresting). Suppose $Iso(M,g)$ is a positive dimensional Lie group. Is it true that for every subgroup ${Id} \neq K \subseteq Iso(M,g)$ , $Met(K)$ is a finite dimensional manifold? Is there a connection between the dimensions of $K$ and $Met(K)$? 

For example we can think on the sphere $\mathbb{S}^n$ with the round metric $g_0$.
It is well known that it is the only $O(n)$-invatiant metric on $\mathbb{S}^n$ (up to scalar multiple), and that $Iso(\mathbb{S}^n,g_0)=O(n)$. So in this case, for $K=Iso(\mathbb{S}^n,g_0)$, $Met(K)=\{\lambda g_0|\lambda > 0\}$ is a one dimensional cone. What happens for smaller $K$? (Do we really need $K$ to be the whole isometry group in order to uniquely determine the metric up to scalar multiple?)

Has these kind of questions been investigated before?

[1]: http://math.stackexchange.com/questions/1229804/under-what-conditions-is-a-linear-automorphism-an-isometry-of-some-inner-product

[2]: http://math.stackexchange.com/questions/1240982/characterisation-of-inner-products-preserved-by-an-automorphism