Asymptotic error of Fourier series partial sum of sawtooth function

In Iwaniec's book, Topics in Classical Automorphic Forms, pg. 4, he gives the statement:

$$\{x\}=\frac{1}{2}-\sum_{n=1}^N\frac{\sin 2\pi nx}{\pi n}+O((1+||x||N)^{-1})$$ where $\{x\}$ denotes the fractional part of $x\in\mathbb R$, and $||x||$ denotes the distance from $x$ to the nearest integer, and the implied constant is "absolute" (which I take to mean that it is independent of $x$ and $N$).

I haven't been able to prove this bound for the error. Does anyone have any ideas how to proceed?

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Hi. I think I finally got this. It's somewhat lengthy but I'll try to write it up just in case anyone is interested. – John M Aug 12 '11 at 15:49

1 Answer

It suffices to prove this for $0 \leq x \leq 1/2$. Let $$g_N(x)=\sum_{n=1}^N\frac{\sin 2\pi nx}{\pi n}+x-\frac{1}{2}$$ To show $g_N(x)=O(\frac{1}{1+xN})$, it suffices to show $g_N(x)$ is $O(\frac{1}{xN})$ and also $O(1)$. But it's hard to show $O(1)$ directly so I'll actually show it is $1/2+O(\log(1+xN))$, which together with $O(\frac{1}{xN})$ implies $O(1)$.

Let $$D_N(t)=1+2\sum_{n=1}^N\cos2 \pi nt=\frac{\sin2\pi(N+1/2)t}{\sin\pi t}$$ be the Dirichlet kernel. Observe that $g_N'(x)=D_N(x)$. Thus: $$g_N(x)=\int_0^xD_N(t)dt - \frac{1}{2}$$

The Dirichlet kernel satisfies the inequalities $|D_N(t)| \leq 2N+1$ and $|D_N(t)|\leq \frac{1}{2|t|}$ for $0<|t|<1/2$. We combine these to get $|D_N(t)| \leq \frac{7}{2}\frac{N}{1+N|t|}$. Then we have: $$\int_0^x|D_n(t)|dt \leq \frac{7}{2}\int_0^x\frac{N}{1+Nt}dt=\frac{7}{2}\log(1+xN)$$ Thus we get $g_N(x)=1/2+O(\log(1+xN))$.

It remains to show that $g_N(x)=O(\frac{1}{xN})$. To get that kind of bound, we need to use the trick of integrating by parts, but we can't do that on $\int_0^xD_N(t)dt$ because the boundary terms will blow up. So we use the fact that $D_N(1-t)=D_N(t)$ and $\int_0^1D_N(t)dt=1$ to rewrite $g_N(x)$ as: $$g_N(x)=-\frac{1}{2}\int_x^{1-x}D_N(t)dt$$ for $0 \leq x \leq 1/2$. Now we can integrate by parts to get:$$\int_x^{1-x}D_n(t)dt=\frac{1}{\pi(N+\frac{1}{2})}\frac{\cos 2\pi(N+\frac{1}{2})x}{\sin\pi x}-\frac{1}{2(N+\frac{1}{2})}\int_x^{1-x}\frac{\cos 2\pi(N+\frac{1}{2})tdt}{\sin^2\pi t}$$

Using the fact that $\sin\pi t \geq 2t$ for $0 \leq t \leq 1/2$, the first term above is bounded by $\frac{1}{2\pi}\frac{1}{(N+1/2)x}$. A similar calculation for the second term gives a bound of $\frac{1}{8}\frac{1}{(N+1/2)x}$. Putting this together, we get: $$|g_N(x)|\leq\left(\frac{1}{4\pi}+\frac{1}{16}\right)\frac{1}{(N+\frac{1}{2})x}$$

Thus, $g_N(x)=O(\frac{1}{xN})$ and so also $O(1)$, thus $g_N(x)=O(\frac{1}{1+xN})$, and we are done.

ADDED LATER: In response to the question about the bound $|D_N(t)| \leq \frac{7}{2}\frac{N}{1+N|t|}$, we consider two cases:

Case 1: When $\frac{1}{2|t|} \leq 2N+1$, we have $\frac{1}{N|t|} \leq 4 + 2/N \leq 6$, since $N$ is a positive integer. Therefore, $$|D_N(t)| \leq \frac{1}{2|t|} \leq \frac{1}{2|t|} \cdot \frac{7}{1+\frac{1}{N|t|}} =\frac{1}{2|t|} \cdot \frac{7N|t|}{1+ N|t|} = \frac{7}{2}\frac{N}{1+N|t|}.$$

Case 2: When $\frac{1}{2|t|} \geq 2N+1$, since $N \geq 1$ we have $$|D_N(t)| \leq 2N+1 \leq \frac{7N}{5N+2}(2N+1) = \frac{7}{2} \frac{N}{1+N/(4N+2)} \leq \frac{7}{2}\frac{N}{1+N|t|}.$$

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John, How did you obtain the bound $|D_N|\le \frac72 \frac{N}{1+N|t|}$ by "combining" the other (easy to obtain) bounds on $D_N$? – Dr. MV Jan 27 at 20:48
@Dr. MV - Please find the answer to your question. – John M Feb 17 at 0:45