For $x\in (-\pi, \pi) $, I want to know whether the following series following series are uniformly convergent on $(-\pi,\pi) $:
1. $\sum _{n=1}^{\infty}{(\frac {x}{n})^n} $
2. $\sum_{n=1}^{\infty}{\frac {1}{((x+\pi)n)^2}}$.
I think series in 2 has to be uniformly convergent for,
$|\frac{1}{((x+\pi)n)^2}|$ $\leq $ $\frac {1}{n^2} $ and as $\sum {\frac {1}{n^2}} $ is convergent, by Wierstrass M-test the series in (2) is uniformly convergent.
But I don't how to proceed for the series in (1). Any help please! Also tell if my answer to the (2)nd is correct.
Which of the following series is uniformly convergent?
633 Views Asked by Bumbble Comm https://math.techqa.club/user/bumbble-comm/detail AtThere are 2 best solutions below
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Number two is not correct: Let $S_{n}(x)=\displaystyle\sum_{k=1}^{n}\dfrac{1}{(x+\pi)^{2}}\dfrac{1}{k^{2}}=\dfrac{1}{(x+\pi)^{2}}\displaystyle \sum_{k=1}^{n}\dfrac{1}{k^{2}}$, $S(x)=\displaystyle\sum_{k=1}^{\infty}\dfrac{1}{(x+\pi)^{2}}\dfrac{1}{k^{2}}=\dfrac{1}{(x+\pi)^{2}}\displaystyle \sum_{k=1}^{\infty}\dfrac{1}{k^{2}}$, then $|S_{n}(x)-S(x)|=\dfrac{1}{(x+\pi)^{2}}\displaystyle \sum_{k\geq n+1}\dfrac{1}{k^{2}}$, if it were uniformly convergent, then for some $N$, we have $|S_{n}(x)-S(x)|<1$ for all $n\geq N$ and $x\in(-\pi,\pi)$, then fix the $n$, take $x\rightarrow-\pi^{+}$ and the expression blows up.
Compliment to @spaceisdarkgreen answer: $\left|\dfrac{\pi}{n}\right|^{n}\leq\left(\dfrac{n^{1/2}}{n}\right)^{n}=\dfrac{1}{n^{n/2}}\leq\dfrac{1}{n^{2}}$ for large $n$, so the series converges.
For part (1) you have $$ \left|\frac{x}{n}\right|^n< \left|\frac{\pi}{n}\right|^n$$ and $$\sum_{n=1}^\infty \left|\frac{\pi}{n}\right|^n$$ converges so the M-test applies here.
Part (2) is funny, cause it seems pretty benign: $$ \sum_{n=1}^\infty \frac{1}{((x+\pi)n)^2} = \frac{1}{(x+\pi)^2}\sum_{n=1}^\infty \frac{1}{n^2} = \frac{\pi^2}{6(x+\pi)}$$ and yet it doesn't uniformly converge since whatever small difference remains from the nicely converging $\sum \frac{1}{n^2}$ is blown up arbitrarily large by the divergence as $x\to -\pi$ (as user284331 shows in detail in their answer).