Radius of Convergence for \sum_{n=2}^{\infty}z^n\log^2(n) in Complex Numbers

In summary, the given series is equal to the sum from n=0 to infinity of z^(n+2)log^2(n+2). By the ratio test, we can determine that the series converges if |z| < 1 and diverges if |z| > 1. This also means that the radius of convergence, R, is equal to 1. There was no need to shift the indices in this case.
  • #1
fauboca
158
0
[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:
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  • #2
Yes, and there was no need to shift the indices.
 
  • #3
LCKurtz said:
Yes, and there was no need to shift the indices.

Thanks.
 

What is the definition of radius of convergence?

The radius of convergence is a mathematical concept that is used in power series to determine the interval in which the series will converge to a finite value. It is denoted by the letter R and is defined as the distance between the center of the power series and the closest point at which the series converges.

How is the radius of convergence calculated?

The radius of convergence is calculated using the ratio test, which involves taking the limit of the absolute value of the ratio of the (n+1)th term to the nth term of the power series. If this limit is less than 1, then the series converges. The radius of convergence is then equal to the reciprocal of this limit.

Why is the radius of convergence important?

The radius of convergence is important because it tells us which values of the variable in the power series will result in a convergent series. It also helps us determine the interval of convergence, which is the range of values for which the series will converge.

What happens if the radius of convergence is infinite?

If the radius of convergence is infinite, it means that the power series will converge for all values of the variable. This is known as a convergent power series. In this case, the interval of convergence is the entire real number line.

Can the radius of convergence be negative?

No, the radius of convergence cannot be negative. It is always a positive value, or it can be infinite. A negative radius of convergence would not make mathematical sense and would not accurately describe the behavior of the power series.

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