What is the Radius of Convergence for the Power Series?

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SUMMARY

The radius of convergence for the power series $\displaystyle\sum_{n=1}^{\infty}\dfrac{(-1)^{n+1}(z-1)^n}{n+1}$ is determined using the ratio test, resulting in a radius of convergence R=1. By substituting s = z - 1, the series simplifies to $\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^{n+1}\ s^{n}}{n+1}$. The ratio of the absolute values of consecutive terms must be less than 1 to find the convergence criteria.

PREREQUISITES
  • Understanding of power series and their convergence
  • Familiarity with the ratio test for series convergence
  • Basic knowledge of complex numbers, specifically in the context of $\mathbb{C}$
  • Ability to manipulate series expressions and substitutions
NEXT STEPS
  • Study the application of the ratio test in greater depth
  • Learn about other convergence tests for power series, such as the root test
  • Explore the implications of radius of convergence on function behavior
  • Investigate the properties of alternating series and their convergence
USEFUL FOR

Mathematicians, students studying complex analysis, and anyone interested in understanding power series and their convergence properties.

Julio1
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Find the radius of convergence of the power series $\displaystyle\sum_{n=1}^{\infty}\dfrac{(-1)^{n+1}(z-1)^n}{n+1}$, $z\in \mathbb{C}.$
Hello !, get as ratio this: $R=\dfrac{1}{|z-1|}.$ And this is equal?
 
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Julio said:
Find the ratio of convergence of the power series $\displaystyle\sum_{n=1}^{\infty}\dfrac{(-1)^{n+1}(z-1)^n}{n+1}$, $z\in \mathbb{C}.$
Hello !, get as ratio this: $R=\dfrac{1}{|z-1|}.$ And this is equal?

First of all, you are finding a RADIUS of convergence using the ratio test.

You need to set the ratio of the absolute value of two consecutive terms less than 1, and then solve for z.

You are close...
 
Julio said:
Find the radius of convergence of the power series $\displaystyle\sum_{n=1}^{\infty}\dfrac{(-1)^{n+1}(z-1)^n}{n+1}$, $z\in \mathbb{C}.$
Hello !, get as ratio this: $R=\dfrac{1}{|z-1|}.$ And this is equal?

Setting s= z-1 the series becomes...

$\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^{n+1}\ s^{n}}{n+1}\ (1)$

... and applying the ratio test You can verify that is R=1...

Kind regards

$\chi$ $\sigma$
 

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