1
$\begingroup$

Does $$f(z)=\displaystyle \sum_{n=0}^{\infty} \frac{2^n+n^2}{3^n+n^3}z^n$$ converge for $z=\frac{-3}{2}$?

  • 0
    In general, text should not go between dollars.2012-06-13
  • 0
    Is this a homework question?2012-06-13
  • 0
    no, I'm studying for GRE test.... I'm trying using Abel's test but I need to clarify this answer2012-06-13
  • 0
    it would be better if you put your solution here and ask for the clarification2012-06-13
  • 0
    To be clear, this is a subquestion of the [OP's previous queston](http://math.stackexchange.com/questions/156280/when-does-sum-n-0-infty-frac2nn23nn3zn-converge)2012-06-13

2 Answers 2

3

Hint (assuming this is homework):

Consider the terms $$\frac{2^n+n^2}{3^n+n^3}\left(\frac{-3}{2}\right)^n.$$

Can you find the limit of this expression as $n\to\infty$?

  • 0
    I Know that this series converges for $|z|<\frac{3}{2}$ by ratio test...and diverges for $z=\frac{3}{2}$ using limit of $\frac{a_n}{a_{n+1}}$.but what about $z=\frac{-3}{2}$2012-06-13
  • 1
    Then ask: Can the series converge if the terms do not converge to zero?2012-06-13
  • 0
    so series diverges for $z=\frac{-3}{2}$2012-06-13
  • 1
    It certainly cannot converge.2012-06-13
  • 0
    Thanks for your hints...this was helpful2012-06-13
1

$$ \frac{2^n + n^2}{3^n + n^3} z^n = \frac{2^n}{3^n + n^3} z^n + \frac{n^2}{3^n + n^3} z^n $$ $$ \frac{n^2}{3^n + n^3} z^n < n^2 \left ( \frac{z}{3} \right )^n = \frac{n^2 }{(-2)^n } \text{ Which converges from Ratio test }$$ $$ \frac{2^n}{3^n + n^3} z^n = \left ( \frac{2}{3} z\right )^n \frac{1}{1 + \frac{n^3}{3^n}} \text{ which is } (-1)^n \text{ for } n \rightarrow \infty \text{ (It does not converge) } $$