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arXiv · 2112.11884

The three missing terms in Ramanujan's septic theta function identity

Örs Rebák
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Abstract

On page 206 in his lost notebook, Ramanujan recorded the following enigmatic identity for his theta function $\varphi(q)$: \begin{equation*} \varphi(e^{-7\pi\sqrt{7}}) = 7^{-3/4}\varphi(e^{-\pi\sqrt{7}})\big\{1 + (\quad)^{2/7} + (\quad)^{2/7} + (\quad)^{2/7}\big\}. \end{equation*} We give the three missing terms. In addition, we calculate the class invariant $G_{343}$ and further special values of $\varphi(e^{-n\pi}),$ for $n = 7, 21, 35,$ and $49$.

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Örs Rebák. 2021-12-22. The three missing terms in Ramanujan's septic theta function identity. https://doi.org/10.1007/s11139-022-00584-2

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