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Homework Help: Radius of convergence

  1. Jan 30, 2012 #1
    [tex]\sum_{n=2}^{\infty}z^n\log^2(n), \ \text{where} \ z\in\mathbb{C}[/tex]

    [tex]\sum_{n=2}^{\infty}z^n\log^2(n) = \sum_{n=0}^{\infty}z^{n+2}\log^2(n+2)[/tex]

    By the ratio test,

    [tex]\lim_{n\to\infty}\left|\frac{z^{n+3}\log^2(n+3)}{z^{n+2}\log^2(n+2)}\right|[/tex]

    [tex]\lim_{n\to\infty}\left|z\left(\frac{\log(n+3)}{ \log (n+2)}\right)^2\right| = |z|[/tex]

    if [itex]|z|<1[/itex], then the sum converges, and if [itex]|z|>1[/itex], then the sum diverges.

    Does this mean that [itex]R=1[/itex]?
     
    Last edited: Jan 30, 2012
  2. jcsd
  3. Jan 30, 2012 #2

    LCKurtz

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    Yes, and there was no need to shift the indices.
     
  4. Jan 30, 2012 #3
    Thanks.
     
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