Prove that $ \int\limits_{-\infty}^{+\infty}\frac{\sin x}{x}\cdot\frac{\sin(x/3)}{x/3}\dots\frac{\sin(x/15)}{x/15}\ dx<\pi $

calculusfourier transformintegration

How to prove the following?
$$
\int\limits_{-\infty}^{+\infty}\frac{\sin x}{x}\cdot\frac{\sin(x/3)}{x/3}\cdot\frac{\sin(x/5)}{x/5}\cdot\frac{\sin(x/7)}{x/7}\cdot\frac{\sin(x/9)}{x/9}\cdot\frac{\sin(x/11)}{x/11}\cdot\frac{\sin(x/13)}{x/13}\cdot\frac{\sin(x/15)}{x/15}\ dx<\pi
$$

First of all, here is the Fourier transform I used:
$$
\hat{f}(y)=F[f(x)]=\int\limits_{-\infty}^{+\infty}f(x)e^{ixy}dx
$$

Well, I tried to solve the problem using convolution. For example, here is the way I used to approach the easier version of the given task $\left(I=\int\limits_{-\infty}^{+\infty}\frac{\sin x}{x}\cdot\frac{\sin(x/3)}{x/3}\cdot\frac{\sin(x/5)}{x/5}\ dx\right)$:
$$
\frac{\sin x}{x}=f(x),\ \ \ \frac{\sin(x/3)}{x/3}\cdot\frac{\sin(x/5)}{x/5}=g(x)\\
I=\frac{1}{2\pi}\int\limits_{-\infty}^{+\infty}\hat{f}(y)\hat{g}(y)dy=\frac{1}{2}\int\limits_{-1}^1\hat{g}(y)dy=\frac{1}{2}\int\limits_{-\infty}^{+\infty}\hat{g}(y)dy-\int\limits_{1}^{+\infty}\hat{g}(y)dy=\pi-\dots\\
\left.
\begin{aligned}
&\hat{g}(y)=\frac{1}{2\pi}\cdot3\pi I_{\left[-\frac{1}{3},\frac{1}{3}\right]}(y)*5\pi I_{\left[-\frac{1}{5},\frac{1}{5}\right]}(y)\\
&\int\limits_{-\infty}^{+\infty}\frac{5}{2}I_{\left[-\frac{1}{5},\frac{1}{5}\right]}(y)dy=1
\end{aligned}
\right\}\Rightarrow\int\limits_{-\infty}^{+\infty}\hat{g}(y)dy=3\pi\int\limits_{-\infty}^{+\infty}I_{\left[-\frac{1}{3},\frac{1}{3}\right]}(y)\ dy=2\pi
$$

I noticed that the integral $J=\int\limits_{1}^{+\infty}\hat{g}(y)\ dy$ doesn't necessarily always equal zero. However, I haven't really managed to prove that the first time $J>0$ is in the initial case (in the given problem). However, I noticed that:
$$\frac{1}{3}+\frac{1}{5}+\dots+\frac{1}{13}<1\\
\frac{1}{3}+\frac{1}{5}+\dots+\frac{1}{15}>1$$

I think that is not a coincidence. Could someone help me to end the solution?

Best Answer

The convolution of the indicator functions extends up to the sum of their widths. That is, the support of the convolution $\circledast_k I_{[-a_k,a_k]}$ is $\left[-\sum_ka_k,\sum_ka_k\right]$. The convolution is a non-negative function, so its integral from $1$ to $\infty$ is positive iff its support extends beyond $1$, that is, if $\sum_ka_k\gt1$.

Related Question