Question: I have asked this question in [math.stackexchange](https://math.stackexchange.com/questions/1711517/a-particular-diophantine-approximation-of-pi-2) without any answer, so I have decided to post it here too.

Recently I was playing around with the sequence $$\frac{1}{n\sin(n)},\ n\in\mathbb{N}.$$

After some computations, I was led to the following question: let $p\_n,q\_n$ be two sequences of natural numbers such that $$\left|\frac{p\_n}{q\_n}-\frac{\pi}{2}\right|<\frac{1}{q\_n^2}.$$

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> Can we find a subsequence of $q\_n$ composed only by even number? Or is it the case that $q\_n$ is odd except for a finite number of indices $n$?
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Any idea or reference is welcome.

Answer: 
The group of connected components of the subgroup of $Aut(X)$ fixing $H^{11}(X)$ is finite. This follows from the main result of 

[Polarized Varieties, Fields of Moduli and Generalized Kummer Varieties of Polarized Abelian Varieties](http://www.jstor.org/stable/2372821),
T. Matsusaka,
American Journal of Mathematics,
Vol. 80, No. 1 (Jan., 1958), pp. 45-82 

I quote from the [mathscinet review](http://www.ams.org/mathscinet-getitem?mr=94360):

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> The main result (section 3) is as follows. Let $U$ be a projectively embeddable non-singular variety; then there is a maximal connected algebraic group $G$ of automorphisms of $U$. Moreover, $G$ is of finite index in the group $G′$ of those automorphisms of $U$ which, for a given projective embedding of $U$, carry a hyperplane section $H$ into a cycle numerically equivalent to $H$.
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Since the group of numerical equivalence classes of divisors embeds in $H^{11}$, the group of automorphisms preserving $H^{11}$ is a subgroup of the group $G'$ above. It is clearly a closed algebraic subgroup, so it likewise has finitely many components.

I must admit, on a quick read, I also endorse the last sentences of the review:

> 
> Unfortunately, the author's style, added to the intrinsic difficulties of the subject, makes it exceedingly hard to check the accuracy of many technical details which seem essential for the validity of the proofs; this is all the more to be regretted, since his results are so valuable and important. It is to be hoped that he, or someone else, will some day give a completely lucid and completely convincing exposition of these topics. 
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Edit: This is just a long comment. 

Let $X$ be a smooth projective variety.

If $\omega\_X$ is ample, then $\mathrm{Aut}(X)$ is finite, so that $G$ is finite. So you see that we should avoid $\omega\_X$ being ample.

If $\omega\_X^\vee$ is ample, then $\mathrm{Aut}(X)$ is not necessarily finite (e.g., $X=\mathbb P^n$), but it's (the $\mathbb C$-points of) an affine finite type group scheme. In particular, its group of connected components is finite. In particular, $G$ is finite here as well.

So, as you're looking for a variety with $G$ infinite, and we should clearly avoid $\omega\_X$ being ample or anti-ample, my guess is that looking at CY-variety might work. [Edit: I now realize that this never works. If $X$ is a CY manifold then the kernel is finite.]

Now, as you know, if $X$ is a curve of genus one (resp. a K3 surface), your representation $\rho$ will have finite kernel (resp. trivial kernel). So, we will have to start considering CY-threefolds.

Now, there are examples of CY-threefolds with infinite automorphism group. The first example that comes to mind is provided by the variety "X" in <http://arxiv.org/pdf/1306.1590v3.pdf> (also mentioned previously on MO: [Can a rigid CY threefold have infinitely many automorphisms](https://mathoverflow.net/questions/207551/can-a-rigid-cy-threefold-have-infinitely-many-automorphisms?rq=1)).

EDIT (thanks for David Speyer for the comments):

My first guess would be that the associated representation has infinite kernel in this case. Unfortunately, it does not. In fact, you will also have to look further than Calabi-Yau manifolds, as Prop. 2.4 in <http://arxiv.org/pdf/1206.1649v3.pdf> shows that the kernel of $\rho$ is always finite in this case as well.