MHB Solving Abs Converge Issue with Ratio Test

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The discussion centers on the application of the ratio test to determine the convergence of a Fourier series. The user believes the series is absolutely convergent based on their calculations, but the book states otherwise. It is clarified that the ratio test is inconclusive when the limit equals 1, which is the case here. The confusion arises from mixing up the ratio test with the limit comparison test, which has different criteria for establishing convergence. Ultimately, the ratio test does not confirm absolute convergence in this scenario.
Dustinsfl
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The book says this isn't absolutely convergent but I keep getting it is by the ratio test. What is wrong?

The Fourier series for $f$ is $f(\theta) = 2\sum\limits_{n = 1}^{\infty}\frac{(-1)^{n + 1}}{n}\sin n\theta$.
Then
$$
-\sum\limits_{n = 1}^{\infty}\left|\frac{(-1)^{n + 1}}{n}\right|.
$$
By the ratio test, we have
\begin{alignat*}{3}
\lim_{n\to\infty}\left|\frac{(-1)^{n + 2}n}{(-1)^{n + 1}(n + 1)}\right| & = & \lim_{n\to\infty}\left|\frac{-n}{n + 1}\right|\\
& = & \lim_{n\to\infty}|-1|\frac{n}{n + 1}\\
& = & 1 < \infty
\end{alignat*}

I solved this problem in another manner but shouldn't I be able to get the same answer using the Ratio Test?
 
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dwsmith said:
I solved this problem in another manner but shouldn't I be able to get the same answer using the Ratio Test?
The ratio test is inconclusive if the limit \(L = 1\). You are perhaps confusing this with the limit comparison test, which only requires the limit to exist and be nonzero in order to establish convergence or divergence.
 
We all know the definition of n-dimensional topological manifold uses open sets and homeomorphisms onto the image as open set in ##\mathbb R^n##. It should be possible to reformulate the definition of n-dimensional topological manifold using closed sets on the manifold's topology and on ##\mathbb R^n## ? I'm positive for this. Perhaps the definition of smooth manifold would be problematic, though.

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