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Convergence of series, sequences and functions and different modes of convergence.

4 votes
0 answers
453 views

Is there any closed form expression for $\sum_{k=2}^\infty(-1)^k \left(- \frac{1}{2}\right)^...

Is there any closed form expression for the following serie? $$\sum_{k=2}^\infty(-1)^k \left(- \frac{1}{2}\right)^{\frac{k(k+1)}{2}}$$ Or at least a proof that it is an irrational number. The contex …
John Finkelstein's user avatar
2 votes
0 answers
1k views

Is there an infinite product like this for $\cos x$?

There are infinite products of iterated square roots for $\log x$ and $\arccos x$ as functions of $x$. For example $$\log x = \frac{x - 1}{\sqrt{x}\sqrt{\frac{1}{2} + \frac{1}{2}\left ( \frac{1 + x}{ …
John Finkelstein's user avatar
1 vote
0 answers
308 views

Can Carlsons's iterative algorithm for $\arctan x$ be inverted to get one for $\tan x$?

In the article An algorithm for computing logarithms and arctangents, by B. C. Carlson, the following iterative algorithm for arctangents is given: The algorithm uses that $2^n\tan(2^{-n}\arctan(x) …
John Finkelstein's user avatar