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I want to show that there is some $\gamma(n)=o(n^{-1})$ and some $C(n) \to -\infty$ such that for $\gamma \leq \theta \leq \pi$ we have

$\sum_{k=1}^n -1+\cos(k\theta) \leq C(n)$.

If we rewrite this using the Dirichlet kernel, what I want is that: if $\gamma \leq \theta \leq \pi$ then

$-n+\frac{D_n(\theta)}{2}-\frac{1}{2} \leq C(n)$.

If I didn't demand that $\gamma=o(n^{-1})$ then I could do this with $\gamma=n^{-3/4}$ say, and $C(n)$ could then just be $-n$.

I've plotted this a bunch and it looks true, and I've spent the morning trying to prove it. I suspect I may have to not just bound things using absolute values, but actually know where things are positive and negative

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The left hand side, when multiplied by $\theta,$ looks awfully like the Riemann sum for $\sin^2 x,$ from which the result should follow. On the other hand, this looks awfully like homework, so voting to close. –  Igor Rivin Sep 15 '12 at 23:33
    
Above multiplied->divided. –  Igor Rivin Sep 15 '12 at 23:33
    
I have to say that your homework detector is broken! –  Jordan Sep 15 '12 at 23:35
    
It's a bit annoying that taking the time to find a nice framing for my question makes it sounds more like homework to some people. –  Jordan Sep 15 '12 at 23:38
1  
@Jordan: It really helps to say where the question comes from. Also, our phrasing suggests that you know this to be true but can't show it, which suggests homework. I make no claim to clairvoyance, but these are two indicators. –  Igor Rivin Sep 16 '12 at 3:43

1 Answer 1

up vote 3 down vote accepted

To elaborate on my comment: First, $\cos k \theta - 1 = -2 \sin^2 \frac 12 k \theta,$ so the inequality can be rephrased as saying that

$\lim_{n\rightarrow \infty} \sum_{k=1}^n \sin^2 k \theta/2 = \infty.$

For $\theta=O(1)$ you already know how to do this. For $\theta \ll 1/n,$ as per @Matt's suggestion one can use the Taylor series approximation: each term is of order $k^2 \theta^2,$ so the sum is of order $\theta^2 n^3,$ which indicates that it goes to infinity for $\theta = O(n^{-3/2}).$ If $\theta = o(1),$ but $\theta=O(1/n),$ use the Riemann sum approximation, to show that the sum is asymptotic to $\frac{1}{\theta} \int_0^{n \theta} \sin^2 x dx.$ The integrand is positive, so is the integral. The upper limit is bigger than some constant, $1/\theta$ grows at least linearly....

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