# Fourier Transform of $\frac{\sin(\|x\|)}{\|x\|}$

I am having a problem finding the Fourier Transform of $$f(x) = \frac{\sin(\|x\|)}{\|x\|}$$

I am interested in the cases where $$x \in \mathbb{R}^n, n= 2, 3$$.

Any help much appreciated !

• Does $n$ refer to the dimension of $x$ or to the norm? – Andy Walls Dec 24 '20 at 19:43
• @AndyWalls It refers to the dimension of x, thanks for the observation, I have added it to the question. – kot Dec 24 '20 at 20:50
• See the comments here. The function $(1 - \rho^2)^{-1/2} [\rho < 1]$ doesn't need to be regularized. – Maxim Dec 24 '20 at 21:06

## 2 Answers

From Bracewell, the n-dimensional Fourier Transform

$$F(s_1, \dots, s_n) = \int_{-\infty}^{\infty} \dots \int_{-\infty}^{\infty} f(x_1,\dots, x_n) e^{-2\pi i (x_1 s_1 + \dots + x_n s_n)} \space dx_1 \dots dx_n$$

when there is n-dimensional symmetry, can be manipulated into

$$F(q) = \dfrac{2\pi}{q^{\frac{1}{2}n-1}} \int_{0}^{\infty} f(r) J_{\frac{1}{2}n-1}(2\pi qr)\space r^{\frac{1}{2}n} \space dr$$

Where $$r$$ and $$q$$ are the radii from the origin in the original domain and transform domain respectively.

For the case of $$n=2$$, you need to compute

$$F(q) = 2\pi \int_0^\infty \dfrac{\sin(r)}{r}J_0(2\pi qr)\,r\, dr = 2\pi \int_0^\infty \sin(r)J_0(2\pi qr) \,dr$$

For the case of $$n=3$$ note that

$$J_{\frac{1}{2}}(x) = \sqrt{\dfrac{2}{\pi x}}\sin(x)$$

so you need to compute

$$F(q) = \dfrac{2\pi}{\sqrt{q}} \int_0^\infty \dfrac{\sin(r)}{r}\sqrt{\dfrac{2}{\pi (2\pi qr)}}\sin(2\pi qr)\,r\sqrt{r}\, dr = \dfrac{2}{q}\int_0^\infty \sin(r)\sin(2\pi qr)\, dr$$

Update

It is worth noting that the n-spherically symmetric Fourier transform (aka Hankel Transform) is its own inverse. Also for the $$n=3$$ case

\begin{align*} f(r) &= \dfrac{2\pi}{\sqrt{r}} \int_0^\infty \dfrac{\delta\left(q-\frac{c}{2\pi}\right)}{2q}\sqrt{\dfrac{2}{\pi(2\pi rq)}}\sin(2\pi rq)q\sqrt{q} \, dq\\ \\ &= \dfrac{1}{r}\int_0^\infty \delta\left(q-\frac{c}{2\pi}\right)\sin(2\pi rq) \, dq\\ \\ &= \dfrac{\sin(cr)}{r}\\ \end{align*}

• Many thanks Andy ! – kot Dec 24 '20 at 21:16
• You're welcome. Also note that the case for $n=3$ looks like it may be spherical delta function, something like $k\dfrac{\delta\left(q-\frac{1}{2\pi}\right)}{q}$, since it looks like the integral may be 0 for $q\ne\frac{1}{2\pi}$. – Andy Walls Dec 24 '20 at 22:17
• Can I do the above analysis also for $\frac{sin(c||x||)}{||x||}$, $c$ a real constant ? – kot Dec 24 '20 at 22:38
• Sure. See my update for the $n=3$ transform pair $\dfrac{\delta\left(q-\frac{c}{2\pi}\right)}{2q} \leftrightarrow \dfrac{\sin(cr)}{r}$ – Andy Walls Dec 24 '20 at 22:47

Formally $$\int_{\mathbb{R}}\int_{\mathbb{R}}\frac{\sin\sqrt{x^2+y^2}}{\sqrt{x^2+y^2}}e^{-isx}e^{-ity}\,dx\,dy =\int_{-\pi}^{\pi}\int_{0}^{+\infty}\sin(\rho)e^{-is\rho\cos\theta}e^{-it\rho\sin\theta}\,d\rho\,d\theta$$ equals $$\int_{-\pi}^{\pi}\frac{d\theta}{(s\cos\theta+t\sin\theta)^2-1}=\int_{-\pi}^{\pi}\frac{d\phi}{(s^2+t^2)\sin^2\phi-1}=\frac{-2\pi}{\sqrt{1-(s^2+t^2)}}$$ and the Fourier transform of a radial function is a radial function, but there are issues in the application of Fubini's theorem since $$\frac{\sin\|x\|}{\|x\|}$$ does not belong to $$L^1(\mathbb{R}^2)$$ or $$L^1(\mathbb{R}^3)$$.