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Complex, contact, Riemannian, pseudo-Riemannian and Finsler geometry, relativity, gauge theory, global analysis.

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From a sum of cosines to an integral of Bessel function

In a physics paper (pubs.acs.org/doi/10.1021/j100210a011), I see the following transformation: $$\sum_q \frac{2[1-\cos(\textbf{q} \cdot \textbf{r})]}{q^2} =\frac{1}{\pi} \int_0^{+\infty}[1-J_0(qr)]\fr …