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There are many elliptic integrals, so to show my point let me just pick one of them (complete elliptic integral of the first kind [1]):

$$K(k) = \int_{0}^{1} \frac {dx} {\sqrt{(1-x^{2})(1-k^{2}x^{2})}}$$

A beautiful solution [1] to this is given by the third Jacobi theta function $\theta_{3}$:

$$K(k) = \frac{\pi}{2} \theta_{3}^{2}(q)$$

for some function $q = q(k)$.

This means that $K(k)$ enjoys some sort of modularity after some clever transformation! And whenever I see a function with (near) modularity, I wonder if it's a function (or a section) on the moduli space of elliptic curves.

Question: Given $k$, can you construct a Riemann surface $\Sigma(k)$ and interpret $K(k)$ as some quantity related to $\Sigma(k)$ (e.g. a period or some sort) so that the modularity of $K(k)$ is a direct corollary? It would be even better if such $\Sigma(k)$ is the surface defined by

$$y^{2} = (1-x^{2})(1-k^{2}x^{2}).$$

Related

Reference

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  • $\begingroup$ For those like me who want to look it up: $q= \exp\bigl(-\pi\, K(\sqrt{1-k^2})/K(k)\bigr)$. $\endgroup$ Commented Dec 1, 2023 at 9:36

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