Diophantine approximation Let $P$ be a polynomial of fixed degree $d$ with integer coefficients of absolute values at most $n$. Assume that $P(\cos 2\pi/n)$ is no zero. Is there a lower bound for $|P(\cos 2\pi/n)|$ ? For instance, is this at least $n^{-C(d) }$ where $C(d)$ is some constant depending on $d$ ? 
 A: Let $\alpha = 2^dP(\cos 2\pi /n)$. Then $\alpha$ is a non-zero algebraic integer and thus its norm is a positive integer. The norm is the product of the $\phi(n)$ conjugates of $\alpha$, all of whom are of absolute value at most $n2^d$, so $|\alpha| \ge (1/n2^d)^{\phi(n)-1}$. This is not as good as you'd like, especially for large $n$. I think the problem with fixed $d$ and large $n$ is hard. See the following question:
How small can a sum of a few roots of unity be?
By the way, did you mean the two occurrences of $n$ to be the same? 
A: It seems one can get something by just looking at the Taylor expansion of $\cos$. Let me consider $\cos 1/n$ for simplicity. Plugging 
$$\cos 1/n = 1- (1/n)^2/2! + (1/n)^4/ 4! -.... $$
into $P= a_d x^d + a_{d-1} x^{d-1}+ ...+ a_1 x + a_0 $ one has 
$$(a_d+ ...+ a_0) - \frac{(1/n)^2}{2!} (da_d +...+ a_1) -....$$
If $a_d+ ...+a_0$ is not zero, it dominates everything else, as $|a_i| \le n$. If it is zero, 
repeat the argument for $da_d+ ...+a_1$ and so on. By linear independency, this cannot go
for more than $d$ steps (I guess). So we should have a lower bound like $n^{-2d}$. 
