Tagged Questions

Convergence of sequences and different modes of convergence.

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Testing convergence of series $\sum_{n=1}^\infty\sin(\pi\sqrt{n^2+k^2})$ [duplicate]

Considering $$\sum_{n=1}^\infty\sin(\pi\sqrt{n^2+k^2})$$ depending on $k$, which can be real. I have absolutely no clue how to proceed. Tried to taylor it, but with no result.
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is there any convergent sub-sequence of a sequence of all rational numbers?

Let $(a_n)$ be a sequence of rational numbers, where all rational numbers are terms. (i.e. enumeration of rational numbers) Then, is there any convergent sub-sequence of $(a_n)$?
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Does there exist a if and only if condition so that arc length of a convergent sequence of functions to converge to the arc length of the limit.

Is there a necessary and sufficient condition so that the arc length of a convergent sequence of functions converges to the arc length of the limit of the function? I know that if $f'_n$ is ...
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Convergence test for series $\sum_{n=2}^{\infty}\frac{(-1)^n}{\sqrt{n}\sqrt{n+(-1)^n}}$

What would be the approach to resolve whether this series converges (absolutely or conditionally) or diverges?
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On series of the kind $\sum_{n=1}^\infty\frac{1}{n^2}\cdot\frac{1}{(1+nx)^s}$ and Frullani's theorem

I would like to know if my computations, in this post are not required justifications for the computations unless if there is a mistake in my claims, were rights and solve a question as if are known a ...
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If the series $\sum_{k=1}^{\infty} a_k$ is Cesàro summable and $n a_n \to 0$ as $n \to \infty$, then the series converges

I'm learning about Fourier series, specifically Cesàro summable sequences and series, and need help with the following problem: Show that if the series $\sum_{k=1}^{\infty} a_k$ is Cesàro ...
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Prove or disprove: $\sum_{n=0}^\infty e^{-|x-n|}$ converges uniformly in $(-\infty,\infty)$

I'm trying to find out if $\sum_{n=0}^\infty e^{-|x-n|}$ converges uniformly in $(\infty,\infty)$. Here's my attempt at an answer: If $x\in(-\infty,0]:$ $$e^{-|x-n|}=e^{(-(x-n))}=e^{x-n}\le e^{-n}$$ ...
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Does this series converge? (test for convergence)

I have a series defined like this: $$\sum_{n=1}^{\infty} (-1)^n (\cos \frac{1}{n})^n$$ and I need to find out whether it converges or diverges. I know that $\lim_{n\to\infty} |a_n| = 1$ but does it ...
Doubts concerning to an application of Frullani's theorem to $f_k(x)=\frac{2^{-k^2x}-2^{-(k^2+1)x}}{x}$, and Lebesgue convergence theorems
By application of Frullani's theorem for $a_n=n^2+1$, $b_n=n^2$ where $n\geq 2$ and $f(x)=2^{-x}$ then RHS in Frullani's integral is obtained for $n\geq 2$ as $$\log(1-\frac{1}{n^2}),$$ thus I asked ...
For $f \in L^1(\mathbb{R})$, we can proof, that $\sum_{n\in \mathbb{Z}} |f(t+na)|<\infty$ ALMOST EVERYWHERE for $t\in [0,a]$ ($\star$ proof below) My Question: If we assume \$f\in L^1(\mathbb{R})\...