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Homework Statement
Describe the set of all [tex]z \in \mathbb{C}[/tex] such that the series [tex]\sum_{n=1}^{\infty} (1-z^2)^n[/tex] converges
Homework Equations
Basic analytic techniques.
The Attempt at a Solution
This is from a graduate complex analysis class, and I just have a feeling my answer is too obvious to be correct...
Basically, I used the necessary condition that the the sequence given by [tex]a_n = |(1-z^2)^n| [/tex] must tend to 0 as n tends to infinity. In other words, we know the modulus of the terms must go to zero. Then, viewing the series as a geometric series, [tex]|(1-z^2)| < 1[/tex]. From there it was a few simple steps to conclude the set is [tex]\lbrace z\in \mathbb{C} \: | \: 0<|z|<2 \rbrace [/tex].
Even as I read over this, it makes sense. But like I said, it's much too simple to be correct...what am I forgetting here?