In his answer to Bitrex's question, Sasha used generalized function properties to derive the Fourier transform of order $0$ Bessel function of the first kind.

I can't work out the logic he used in his answer.
Can someone please clarify this for me?
Maybe mention the identities used in the derivation of the answer?
I'm Mainly puzzled by the transition to the third row, and the notation in the following, fourth row: −1≤ω≤1 .

  • $\begingroup$ Do you've a particular step you can't wrap your head around or is it thw whole derivation you don't understand? Right now it's not clear what exactly you're looking for. $\endgroup$ – mrtaurho Oct 9 at 17:50
  • $\begingroup$ Mainly from the second to third line. $\endgroup$ – Gil Oct 9 at 17:52
  • $\begingroup$ So the step where the Dirac Delta is introduced? $\endgroup$ – mrtaurho Oct 9 at 17:53
  • $\begingroup$ Yes, Is this an Identity proven somewhere? or a logical conclusion? also in the following line the notation of omega between -1 and 1 makes no sense to me... $\endgroup$ – Gil Oct 9 at 17:56

In a distributional sense the following identity holds

$$\frac1{2\pi}\int_{\Bbb R}e^{ikx}\mathrm dx=\delta(k)$$

This is a standard result (I found this PDF after a quick search proving the identitiy and explaining the necessitiy of this result). In the linked answer $k=\omega+\sin\tau$.

${\bf 1}_{-1\leq\omega\leq1}$ is a so-called characteristic function. This one simply translates to

$${\bf 1}_{-1\leq\omega\leq1}=\begin{cases}1&,\text{if }-1\leq\omega\leq1\\ 0&,{\text{else}}\end{cases}$$

I am not completey sure but it seems to me like ${\bf 1}_{-1\leq\omega\leq1}$ is introduced for reasons of convergence in the linked answer.

  • $\begingroup$ Thank you, that helps. $\endgroup$ – Gil Oct 9 at 18:25
  • $\begingroup$ @Gil Glad to do so! $\endgroup$ – mrtaurho Oct 9 at 18:26

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