Complex Series, Region of Convergence

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Discussion Overview

The discussion revolves around determining the region of convergence (ROC) for a complex series involving the term Ʃ(0,inf) (n(n-1)(z+5i)^n)/n. Participants are exploring the application of the ratio test and addressing potential missteps in the calculation of limits.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant questions their application of the ratio test and expresses uncertainty about the resulting limit.
  • Another participant challenges the simplification of the limit to an indeterminate form (inf/inf) and emphasizes the need to evaluate the limit more carefully.
  • A participant suggests that the ROC can be expressed as |z+5i| < inf, although this statement is not fully elaborated.
  • There is a prompt for further clarification on what the limit must be smaller than for convergence, indicating a need for additional exploration of the conditions for convergence.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct approach to solving the problem, and multiple viewpoints regarding the application of the ratio test and the interpretation of the limit remain unresolved.

Contextual Notes

There are limitations in the discussion regarding the assumptions made in applying the ratio test and the interpretation of the limit, which could affect the conclusions about the region of convergence.

Who May Find This Useful

This discussion may be useful for students and practitioners in electrical engineering, mathematics, or related fields who are interested in series convergence and the application of convergence tests in complex analysis.

sgonzalez90
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Good evening, I'm an electrical engineering student questioning my answer to this series Region of Convergence problem.

Ʃ(0,inf) (n(n-1)(z+5i)^n)/n

Using the ratio test lim n-> |an+1/an|

I was able to get it down to lim n->|n(z+5i)/(n-1)| which gave |inf/inf| = 1, which means the test fails. What do I do to properly solve this? :(
 
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you can't just write it as inf/inf and cancel them!
[tex]\lim_{n\to \infty} |\frac{n(z+5i)}{n-1}| = |z+5i| \lim_{n\to \infty} \frac {n}{n-1}[/tex]
 
ROC
= |z+5i| < inf?

:D
 
Once you know the limit is |z+5i|, what does this have to be smaller than for the series to converge?
 

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