Using the ratio test to figure if the series is convergent

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Homework Help Overview

The discussion revolves around the convergence of the series \(\sum_{n=1}^{\infty} \frac{(-2)^n}{n^n}\), with participants exploring the application of the ratio test to determine its behavior.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants attempt to apply the ratio test and express limits involving the series terms. There are questions about the presence of brackets and the implications of growth rates between \(n^n\) and \(n!\). Some participants express uncertainty about handling numerical factors when simplifying limits.

Discussion Status

The discussion includes various attempts to analyze the series using the ratio test, with some participants suggesting that the series of positive terms converges. There is no explicit consensus, but several lines of reasoning are being explored regarding the convergence of the series.

Contextual Notes

Some participants note potential missing information or assumptions in the problem setup, particularly regarding the handling of terms in the limit process.

Aerospace93
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Homework Statement


\sum ftom n=1 to \infty (-2)n/nn.

The Attempt at a Solution


limn->\infty | (-2)n+1/(n+1)n+1) x nn/(-2)n | = |-2|limn->\infty |(n/n+1)n*(1/n+1) |

If it were only (n/n+1) then would the answer be 2e? Either way, how do you sole this the way it is?
 
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I think there are brackets missing.
(n/(n+1)^n) is smaller than 1, that is sufficient here. The other factor is more interesting.
 
Another way to look at it: The growth rate of n^n is faster than that of n!. If you replace n^n with n!, where does that leave you at?
 
Aerospace93 said:

Homework Statement


\sum ftom n=1 to \infty (-2)n/nn.

The Attempt at a Solution


limn->\infty | (-2)n+1/(n+1)n+1) x nn/(-2)n | = |-2|limn->\infty |(n/n+1)n*(1/n+1) |

If it were only (n/n+1) then would the answer be 2e? Either way, how do you sole this the way it is?

If n > 3 then (2/n)^n < (2/3)^n.
 
Aerospace93 said:

Homework Statement


\sum ftom n=1 to \infty (-2)n/nn.

The Attempt at a Solution


limn->\infty | (-2)n+1/(n+1)n+1) x nn/(-2)n | = |-2|limn->\infty |(n/n+1)n*(1/n+1) |

If it were only (n/n+1) then would the answer be 2e? Either way, how do you sole this the way it is?

Notice that by the ratio test, the series of positive terms will converge and so Ʃan will be absolutely convergent which implies Ʃan will converge.
 
Zondrina said:
Notice that by the ratio test, the series of positive terms will converge and so Ʃan will be absolutely convergent which implies Ʃan will converge.

By the ratio test I'm left with: 2 x limx->\infty |nn/(n+1)n+1)| which is the same as what i previously wrote: 2 x limx->\infty [n/(n+1)]n*1/(n+1).

Dividing by the largest power of the polynomial in the denominator:
2 x limx->\infty [1/(1+1/n)]n*1/(n+1).-
so it becomes: 2 x limx->\infty |1n*[1/(1+n)] = 2 x limx->\infty 1n*0=0
ratio test = 0>1 therefore the series is absolutely convergent and therefore convergent.

The only thing I am not sure about doing in this problem really is how to deal with the numbers when i divide them by n when I'm working out the limit.
 
Aerospace93 said:
The only thing I am not sure about doing in this problem really is how to deal with the numbers when i divide them by n when I'm working out the limit.
You can always divide numerator and denominator by n, if you do it consistently (as n!=0).
 

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