# Duplication Formula for Weierstrass's Elliptic Function

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## Theorem

Let $\omega_1$, $\omega_2$ be non-zero complex constants with $\dfrac {\omega_1} {\omega_2}$ having a positive imaginary part.

Let $z$ be a complex number where $z \notin \set {2 m \omega_1 + 2 n \omega_2: \tuple {n, m} \in \Z^2}$.

Then:

- $\map \wp {2 z; \omega_1, \omega_2} = \dfrac 1 4 \paren {\dfrac {\map {\wp''} {z; \omega_1, \omega_2} } {\map {\wp'} {z; \omega_1, \omega_2} } }^2 - 2 \map \wp {z; \omega_1, \omega_2}$

where:

- $\wp$ is Weierstrass's elliptic function
- $\wp'$ and $\wp''$ denote its first and second derivative with respect to $z$.

## Proof

## Sources

- 1920: E.T. Whittaker and G.N. Watson:
*A Course of Modern Analysis*(3rd ed.): $20.311$: The duplication formula for $\wp \left({z}\right)$