This is a follow up on [the MO question here](https://mathoverflow.net/questions/263482/sinc-ing-integral). I kept being fascinated and bemused by these functions.


Denote $\text{sinc}(x)=\frac{\sin x}x$. Experiments suggest that
$$\sum\_{n=1}^{\infty}\text{sinc}^j\left(\frac{n}{\pi}\int\_{\mathbb{R}}\text{sinc}^j(x)\,dx\right)=\frac{\pi}2-\frac12. \tag1$$



> 
> **Question.** Equation (1) seems to hold true for $j\leq 82$. Further computations done by Pietro indicate the same beyond $j=82$. Is it always true for any $j$?
> 
> 
> 



more on "sinc-ing" integrals and sums
Results like this hold for small values of $j$, but are eventually false. Put $f(x)$ to be the indicator function of the interval $[-1/2,1/2]$, and put $f\_j(x)$ to be the convolution of $f$ with itself $j$ times. Then the Fourier transform of $f\_j$ is 
$$ 
{\hat f\_j}(\xi) = \int\_{-\infty}^{\infty} f\_j(x) e^{-2\pi i x\xi} dx = 
\Big( \int\_{-1/2}^{1/2} e^{-2\pi i x\xi } dx \Big)^j = \Big(\frac{\sin (\pi \xi)}{\pi \xi} \Big)^j = (\text{sinc}(\pi \xi))^j.
$$ 


So the sum in the question is 
$$ 
\sum\_{n=1}^{\infty} {\hat f\_j}(n \alpha\_j), \tag{1}
$$ 
with 
$$ 
\alpha\_j = \frac{1}{\pi^2} \int\_{{\Bbb R}} (\text{sinc}(x))^j dx.
$$ 


Since $\text{sinc}$ is even, and with the usual convention that its value at $0$ is $1$, the quantity in (1) is 
$$ 
-\frac 12 + \frac 12 \sum\_{n\in {\Bbb Z}} {\hat f}\_j(n\alpha\_j)= -\frac 12 + \frac{1}{2\alpha\_j} \sum\_{k} f\_j(k/\alpha\_j), \tag{2}
$$ 
by the Poisson summation formula. 


The contribution of the term $k=0$ above is 
$$ 
\frac{1}{2\alpha\_j} f\_j(0) = \frac{1}{2\alpha\_j} \int\_{-\infty}^{\infty} (\text{sinc}(\pi x))^j dx = \frac{\pi}{2}. 
$$
This explains the purported identity: the function $f\_j$ is supported in $[-j/2,j/2]$ and for small $j$ one has 
$$ 
\frac{1}{\alpha\_j} \ge \frac{j}{2},
$$ 
so that all terms with $k\neq 0$ in (2) vanish. But it is easy to see that $\alpha\_j$ decreases like constant$/\sqrt{j}$, so eventually non zero values of $k$ will contribute, and there will be no exact identity.


