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[tex]\int \frac{d^3 \texbf{q}}{(2 \pi)^3} \frac {e^{i \texbf{q} \dot \texbf{r}}} {q^2 + K^2} = \frac {e^{-Kr}} {4 \pi r}[/tex]

**How do we get from the left hand side to the right hand side?**

I've tried regular Fourier transform of the function under the complex exponential I think that gives 1/(r^2 + K^2) which is nothing like the right hand side... also I thought about doing integration by parts but I only know the normal version of that so I wouldn't know how to tackle a triple integral like this.

Any help would be much appreciated.