Convergence/divergence of the series $\sum_{n=4}^\infty\frac{(-1)^n}{\log(\log(n))}$?

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We have the following sum:

$$\displaystyle \sum_{n=4}^\infty \dfrac{(-1)^n}{\log(\log(n))} $$

I have a hunch this series is conditionally convergent, but I get nowhere using the ratio test. What test would be best to apply to this particular series?

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You may just use the alternating series test:

  • the function $x \mapsto \dfrac{1}{\log(\log(x))}$ is decreasing,

  • as $x \to \infty$, you have $\dfrac{1}{\log(\log(x))} \to 0.$