Let $f(t)=1-t^2$ , for $|t|<1$ and $0$ elsewhere.
Compute the Fourier transform of $f(t)$ and use the result to find the value of the integral
$$
\int_{-\infty}^{\infty}\frac{\sin t-t \cos t}{t^3}dt
$$
SOLUTION:
So the Fourier transform is pretty easy and I got $\hat{f}(\omega)= 4 \frac{\sin \omega- \omega \cos \omega}{\omega^3}$. How do I use this to compute the integral? Since there is an obvious connection between these, can I use the inverse theorem or Parseval's formula?
Best Answer
I solved it myself, here is my solution if anyone else counter the same problem.
So I used Parseval's formula
$$ \int_{-\infty}^{\infty}f(x)\overline{g(x)}dx = \frac{1}{2\pi}\int_{-\infty}^{\infty}\hat{f}(\omega) \overline{\hat{g}(\omega)}d\omega $$ and then I just took $\overline{\hat{g}(\omega)} = 1$ with $g(t) = \delta(t)$, combining this I simply got
$$ \int_{-\infty}^{\infty} \frac{\sin t- t \sin t}{t^3}dt = \frac{2 \pi}{4} \int_{-1}^{1}(1-t^2)\delta(t)dt = \frac{\pi}{2}\,f(0) = \frac{\pi}{2} $$