Radius of Convergence for Series with (-1)^(n-1) and x^n Terms

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Homework Help Overview

The discussion revolves around finding the radius of convergence for the series \(\sum_{n=1}^{\infty} \frac{(-1)^{n-1} x^n}{n}\) and determining a formula for the series at the right-hand endpoint. The subject area is series convergence, specifically focusing on alternating series and power series.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss using the ratio test or root test to find the radius of convergence. There is uncertainty about the presence of the term \((-1)^{n-1}\) and its implications. Some participants suggest recognizing the series as a familiar function and exploring its derivative to gain insights.

Discussion Status

The conversation is ongoing, with participants offering guidance on applying the ratio test and recognizing the series structure. There is an exploration of the relationship between the original series and its derivative, with no explicit consensus reached yet.

Contextual Notes

Some participants note the need to consider absolute convergence and the implications of the alternating series test. There is also mention of the requirement to find a constant when integrating to determine the function associated with the series.

tracedinair
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Homework Statement



Find the radius of convergence of [tex]\sum[/tex] n=1 to ∞ of [(-1)^(n-1) x^(n)/(n)] and give a formula for the value of the series at the right hand endpoint.

Homework Equations




The Attempt at a Solution



Not really sure how to start this. I know I'm supposed to use the root test or the ratio test and then probably Abel's Thm for series. but the n-1 in the series is throwing me off. I've never seen that before. Any help is appreciated.
 
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Your series is just f(x)=x-x^2/2+x^3/3+... Just use the ratio test to find the radius of convergence. To find the actual value at the endpoint you could just ask yourself is you have seen that kind of series before. If it doesn't look familiar, try finding f'(x) and see if that series looks familiar.
 
Dick said:
Your series is just f(x)=x-x^2/2+x^3/3+... Just use the ratio test to find the radius of convergence. To find the actual value at the endpoint you could just ask yourself is you have seen that kind of series before. If it doesn't look familiar, try finding f'(x) and see if that series looks familiar.

So I took f'(x) = 1 - x + x^2 - x^3 + ... so, the endpoint is going to look like x^(n+1)? I need a formula for the value at the right hand endpoint..would that work?
 
No, you have to recognize the series, my point to looking at f'(x) was that it's geometric, it's 1/(1+x). if that's f'(x), what's f(x)?
 
If f'(x) = 1/(1+x) then f(x) = ln(x+1)?
 
ln(x+1)+C, you mean. You have to fix the constant C by finding f(0). But yes, that's the idea.
 
Alright, thank you.
 
tracedinair said:

Homework Statement



Find the radius of convergence of [tex]\sum[/tex] n=1 to ∞ of [(-1)^(n-1) x^(n)/(n)] and give a formula for the value of the series at the right hand endpoint.

Homework Equations




The Attempt at a Solution



Not really sure how to start this. I know I'm supposed to use the root test or the ratio test and then probably Abel's Thm for series. but the n-1 in the series is throwing me off. I've never seen that before. Any help is appreciated.
The ratio test applies to a positive series and a power series converges absolutely inside its "radius of convergence".

Take the absolute value (drop the "[itex](-1)^{n-1}[/itex]") and then use the ratio test.
 

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