Absolute Power Series: Questions & Solutions

In summary: If you don't take the absolute value and the series has either alternating terms or at least infinitely many positive and negative terms, then the limit won't exist - unless it's 0. The test is better with the absolute value.
  • #1
Neon32
68
1
I've 2 questions
1) Why do we take absolute of the power series?
2) I don't get why the interval of convergence is from -inifinity to +infinity. You can find the problem below.

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  • #2
For your first question, look up the "ratio test".

If the series converges for all ##x## then by definition the radius of convergence is ##\infty##.
 
  • #3
Neon32 said:
I've 2 questions
1) Why do we take absolute of the power series?
That is called the ratio test of convergence. (https://en.wikipedia.org/wiki/Ratio_test) The absolute value is all you need to test convergence.
2) I don't get why the interval of convergence is from -inifinity to +infinity.
It converges if the absolute value of the limit is less than 1. In this case, the limit of the ratio is 0 regardless of the value of x. So it converges for any value of x.
 
  • #4
FactChecker said:
That is called the ratio test of convergence. (https://en.wikipedia.org/wiki/Ratio_test) The absolute value

I've seen different problems with the ratio test and they didn't use absolute but when it comes to power series, they use it. Why?
 
  • #5
Neon32 said:
I've seen different problems with the ratio test and they didn't use absolute but when it comes to power series, they use it. Why?

If you don't take the absolute value and the series has either alternating terms or at least infinitely many positive and negative terms, then the limit won't exist - unless it's 0. The test is better with the absolute value.
 
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  • #6
Neon32 said:
I've seen different problems with the ratio test and they didn't use absolute but when it comes to power series, they use it. Why?
A power series is a series in powers of a variable such as x. The Ratio Test requires that all terms be positive, but with a variable, some terms could be negative, if x is negative. The problems you've seen that didn't use absolute values were almost certainly series in which all the terms were positive, such as ##\sum_{n = 1}^\infty \frac 1 {n^2 + 1}##.
 
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Related to Absolute Power Series: Questions & Solutions

1. What is the "Absolute Power Series: Questions & Solutions" series?

The "Absolute Power Series: Questions & Solutions" series is a collection of mathematical problems and their corresponding solutions that focus on the concept of absolute power, which is the power of a number raised to a specific exponent without regard to its sign.

2. Who is the target audience for this series?

This series is designed for anyone interested in learning more about absolute power and its applications in mathematics, including students, teachers, and self-learners.

3. Are there prerequisites for understanding this series?

A basic understanding of algebra, including exponents and equations, is recommended for understanding this series. Some familiarity with calculus may also be helpful, but it is not required.

4. How can this series benefit me as a student or mathematician?

This series can help improve your understanding and problem-solving skills in relation to absolute power, which is an important concept in many areas of mathematics such as calculus, number theory, and algebra. It can also serve as a valuable resource for practicing and reinforcing your knowledge.

5. Is this series comprehensive or is it limited to specific topics?

This series covers a wide range of topics related to absolute power, including its properties, applications, and real-world examples. However, it is not intended to be a comprehensive resource and may not cover every aspect of absolute power in depth.

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