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I am looking for a reference for tables of Hankel/spherical Bessel tranforms. In particular, I'm trying to calculate transforms like \begin{align} f_{LM}(r) & = i^L \sqrt{\frac{2}{\pi}} \int_0^\infty k^2\ dk\ j_L(k r)\ \tilde f_{LM}(k) \\ & = i^L \frac{1}{\sqrt{r}} \int_0^\infty k\ dk\ \sqrt{k} J_{L+1/2}(kr)\ \tilde f_{LM}(k), \end{align} where $j_L(kr)$ is a spherical Bessel function and $J_\nu(kr)$ is a (cylindrical) Bessel function. These are the transforms associated with Fourier transforming a function which has been decomposed into multipoles: \begin{gather} \begin{aligned} f(r,\theta_r,\phi_r) &= \sum_{LM} f_{LM}(r) Y_{LM}(\theta_r,\phi_r), & \tilde{f}(k,\theta_k,\phi_k)&=\sum_{LM}\tilde{f}(k) Y_{LM}(\theta_k,\phi_k), \end{aligned} \\ f(r,\theta_r,\phi_r) = \int \frac{d^3k}{(2\pi)^{3/2}} e^{i \mathbf{k}\cdot\mathbf{r}} \tilde{f}(k,\theta_k,\phi_k). \end{gather} I have tried using Mathematica's HankelTransform function, but it seems to often miss the singular pieces. For instance, with $L=0$, both $$\tilde f_{00}(k) = \frac{1}{k^2+m^2} \quad \text{and} \quad \tilde f_{00}(k) = \frac{-k^2/m^2}{k^2+m^2}$$ give a transform of $$f_{00}(r) = \sqrt{\frac{\pi}{2}}\frac{e^{-m r}}{r},$$ while the second function should give $$f_{00}(r) = \sqrt{\frac{\pi}{2}}\frac{e^{-m r}}{r} - \sqrt{\frac{\pi}{2}}\frac{\delta(r)}{m^2 r^2}.$$

kc9jud
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  • @Aruralreader I just took a look at that -- it seems only to include transforms where the integral actually converges. I guess I'm looking for a reference that goes further and includes pairs with (tempered?) distributions. I added distribution-theory as a tag. – kc9jud Apr 14 '23 at 21:19
  • There should be no $1/r^2$ in your last equation, right? – LL 3.14 Apr 15 '23 at 08:29
  • @LL3.14 I believe there should be -- without it, the inverse transform (of that term) is $$\tilde f_{00}(k) = -\frac{1}{m^2} \int_0^\infty r^2 dr\ j_0(kr) \delta(r) = 0,$$ so it has no effect. If you include the $1/r^2$, that term adds a contribution $$\tilde f_{00}(k) = -\frac{1}{m^2} \int_0^\infty dr\ j_0(kr) \delta(r) = -\frac{1}{m^2},$$ which combines with the transform of the first term to give $$\frac{1}{k^2+m^2} - \frac{1}{m^2} = \frac{-k^2/m^2}{k^2+m^2}.$$ – kc9jud Apr 16 '23 at 02:30
  • Oh ok, I thought it was the Fourier transform at this point. In terms of Fourier transform, what do you want to compute? – LL 3.14 Apr 16 '23 at 12:48
  • @LL3.14 in terms of the Fourier transform, I want $$ f_{LM}(r) Y_{LM}(\theta_r,\phi_r) = \int \frac{d^3k}{(2\pi)^{3/2}} e^{i \mathbf{k}\cdot\mathbf{r}} \tilde f_{LM}(k) Y_{LM}(\theta_k,\phi_k)$$ for given $\tilde f_{LM}(k)$. – kc9jud Apr 16 '23 at 19:59
  • @LL3.14 I just found your related answers here and here -- is the lack of a common definition of the "radial part" of the Dirac delta why I'm having trouble finding a table? – kc9jud Apr 16 '23 at 20:15

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