I was wondering why this works and if it works every time

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Homework Help Overview

The original poster is investigating the convergence of several series involving powers of x and factorial terms, specifically focusing on the series \(\sum n x^{n}\), \(\sum \frac{x^{n}}{n}\), and \(\sum n^{n} x^{n}\). They are exploring the conditions under which these series converge.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to apply the ratio and root tests to determine the convergence of the series and questions the validity of their findings. They also inquire about the general applicability of these tests beyond power functions.

Discussion Status

Some participants confirm the original poster's findings regarding the convergence conditions and suggest that the ratio and root tests can be applied to any series, although they note that the ease of application may vary depending on the series' structure. There is a general acknowledgment of the advice given, but no explicit consensus is reached.

Contextual Notes

The original poster's inquiry is framed within the context of homework constraints, and they are seeking clarification on the application of convergence tests for different types of series.

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Homework Statement


I have to find for which values of x the following converge


Homework Equations


\sum n x^{n}

\sum \frac{x^{n}}{n}

\sum n^{n} x^{x}


The Attempt at a Solution



I used the ratio test for the first two and the root test for the last and found respectively that x must lie within

(-1,1)
[-1,1)
{0}

Are these right?
and
Is this the right way to go all the time?

I was wondering when I'll be able to apply the ratio/root tests for radii of convergence. Is it just when we have power functions?
 
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For the last one you mean sum(n^n*x^n), right? I think they are all correct. You can apply any legitimate test to any series, whether it is a power function or not.
 
The ratio and root test work for any series, as Dick said, but it may be difficult to calculate the ratios or roots. It is when you have products or powers, as in power series, that they are easy to apply.
 
You are correct Dick.

And thanks for the advice guys. =D
 

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