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Fix the same problem again
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ceciliachan1979 committed Mar 5, 2024
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8 changes: 4 additions & 4 deletions Books/BabyRudin/Chapter03/ex06.cecilia.tex
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Expand Up @@ -49,12 +49,12 @@ \subsubsection*{Part d}
\begin{eqnarray*}
& & \frac{\frac{1}{|z^{n+1}| - 1}}{\frac{1}{|z^n| - 1}} \\
&=& \frac{|z^n| - 1}{|z^{n+1}| - 1} \\
&<& \frac{|z^n| - 1}{|z^{n+1}|} \\
&=& \frac{|z^n|}{|z^{n+1}|} - \frac{1}{|z^{n+1}|} \\
&=& \frac{1}{|z|} - \frac{1}{|z^{n+1}|}
&<& \frac{|z^n| - 1}{|z^{n+1}| - |z|} \\
&=& \frac{1}{|z|} \\
&<& 1
\end{eqnarray*}

So, as $ n $ tends to $ \infty $, the sequence $ \frac{a_{n+1}}{a_n} $ is upper bounded by a sequence that tends to $ \frac{1}{|z|} < 1 $, so the sequence converges by the ratio test.
Therefore the $ \limsup \left| \frac{\frac{1}{|z^{n+1}| - 1}}{\frac{1}{|z^n| - 1}} \right| < 1$.

On the other hand, if $ |z| \ge 1 $.

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