Radius of convergence question

In summary, the radius of convergence for a power series is a value, denoted by R, that determines the interval of x-values for which the series will converge. It can be found using the ratio test or the root test. If the value of x is outside the radius of convergence, the series will diverge and not have a finite sum. The radius of convergence cannot be negative and directly affects the convergence of the series, as a larger radius allows for more x-values for which convergence will occur.
  • #1
theBEAST
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Homework Statement


Suppose that the following series converges when x = -4 and diverges when x = 6.
∑{n=0 -> ∞} c_n • x^n

What is the interval of convergence?

The Attempt at a Solution


I think it is [-5,5) but my friend reckons that it is [-5,6). I don't think [-5,6) is correct because this interval would tell us that 5 is a point of convergence but we don't know that... Am I right?
 
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  • #2
You don't have enough information. All you know for sure is you have a radius of convergence at least 4 and not more than 6.
 

What is the radius of convergence for a power series?

The radius of convergence for a power series is a value that determines the interval of x-values for which the series will converge. It is denoted by R and can be found by using the ratio test or the root test.

How do you find the radius of convergence for a power series?

To find the radius of convergence for a power series, you can use the ratio test or the root test. These tests involve taking the limit as n approaches infinity of the absolute value of the ratio or root of the n+1 term to the n term in the series. If this limit is less than 1, the series converges and the radius of convergence is the value of x for which the limit is equal to 1.

What happens when the value of x is outside the radius of convergence?

If the value of x is outside the radius of convergence, the power series will diverge and not converge to a specific value. This means that the series will not have a finite sum and may oscillate or grow without bound.

Can the radius of convergence be negative?

No, the radius of convergence cannot be negative. It is always a non-negative value or can be infinite. A negative radius of convergence would not make sense in the context of a power series as it represents the distance from the center of convergence to the boundary of the interval where the series converges.

How does the radius of convergence affect the convergence of a power series?

The radius of convergence directly affects the convergence of a power series. If the value of x is within the radius of convergence, the series will converge. If it is outside the radius, the series will diverge. The larger the radius of convergence, the more x-values for which the series will converge.

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