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for questions about sequences and series, e.g. convergence, closed form expressions, etc. Note that there is a different tag for spectral sequences, and also note that MathOverflow is not for homework. Please consider consulting the online encyclopedia for integer sequences, if you are trying to identify a given sequence that you have found in your research.

15 votes

Sum of series $a^{i^2}$

This sum equals exactly to: $$ \frac{1}{2}\left(\theta_3 (0,a) -1 \right),$$ and $\theta_3$ is the Jacobi $\theta$ - function.
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5 votes

Is this a rational function?

In Prudnikov, Brychkov, Marichev, vol. 1 there is an explicit formula via basic hypergeometric functions. Not rational.
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2 votes

Hypergeometric sum specific value

You may use the general formula from Brychkov, Prudnikov, Marichev, Integral and Series, Vol.3: $$ _{2}F_{1}(1,1;\frac{1}{2};x)=(1-x)^{-1}\left(1+\frac{\sqrt{x}\arcsin{\sqrt{x}}}{\sqrt{1-x}}\right) $$ …
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