Power Series Convergence and Divergence

Oct 13, 2025 by wbrum.org 143 views

Introduction

Power series are a fundamental concept in mathematics, particularly in calculus and analysis. They are used to represent functions as an infinite sum of terms, each of which is a power of the variable. In this article, we will explore the power series $S(x) = \sum_{n=1}^{\infty} a_n(x-7)^n$ , which converges at $x = 9$ and diverges at $x = 14$ . We will examine the properties of this series and determine which of the given statements must be true.

Power Series Convergence and Divergence

A power series is a series of the form $\sum_{n=0}^{\infty} a_n(x-c)^n$ , where $a_n$ are coefficients, $x$ is the variable, and $c$ is a constant. The series converges or diverges depending on the value of $x$ . If the series converges at a point $x = a$ , it means that the series $\sum_{n=0}^{\infty} a_n(a-c)^n$ converges. Similarly, if the series diverges at a point $x = b$ , it means that the series $\sum_{n=0}^{\infty} a_n(b-c)^n$ diverges.

The Given Power Series

The given power series is $S(x) = \sum_{n=1}^{\infty} a_n(x-7)^n$ . This series converges at $x = 9$ and diverges at $x = 14$ . We can rewrite the series as $S(x) = \sum_{n=1}^{\infty} a_n(x-7)^n = a_1(x-7) + a_2(x-7)^2 + a_3(x-7)^3 + \ldots$ .

Properties of the Power Series

Since the series converges at $x = 9$ , we know that the series $\sum_{n=1}^{\infty} a_n(9-7)^n = \sum_{n=1}^{\infty} a_n(2)^n$ converges. Similarly, since the series diverges at $x = 14$ , we know that the series $\sum_{n=1}^{\infty} a_n(14-7)^n = \sum_{n=1}^{\infty} a_n(7)^n$ diverges.

Analyzing the Statements

We are given four statements to evaluate:

A. The series diverges at $x = 0$ . B. The series converges at $x = 7$ . C. The series diverges at $x = 10$ . D. The series converges at $x = 12$ .

We will analyze each statement separately.

Statement A: The series diverges at $x = 0$

To determine if the series diverges at $x = 0$ , we need to evaluate the series $\sum_{n=1}^{\infty} a_n(0-7)^n = \sum_{n=1}^{\infty} a_n(-7)^n$ . Since the series diverges at $x = 14$ , we know that the series $\sum_{n=1}^{\infty} a_n(14-7)^n = \sum_{n=1}^{\infty} a_n(7)^n$ diverges. However, this does not necessarily mean that the series $\sum_{n=1}^{\infty} a_n(-7)^n$ diverges. In fact, the series $\sum_{n=1}^{\infty} a_n(-7)^n$ may converge or diverge depending on the values of the coefficients $a_n$ . Therefore, we cannot conclude that the series diverges at $x = 0$ .

Statement B: The series converges at $x = 7$

To determine if the series converges at $x = 7$ , we need to evaluate the series $\sum_{n=1}^{\infty} a_n(7-7)^n = \sum_{n=1}^{\infty} a_n(0)^n$ . Since any power of zero is zero, the series $\sum_{n=1}^{\infty} a_n(0)^n$ is equal to zero. Therefore, the series converges at $x = 7$ .

Statement C: The series diverges at $x = 10$

To determine if the series diverges at $x = 10$ , we need to evaluate the series $\sum_{n=1}^{\infty} a_n(10-7)^n = \sum_{n=1}^{\infty} a_n(3)^n$ . Since the series diverges at $x = 14$ , we know that the series $\sum_{n=1}^{\infty} a_n(14-7)^n = \sum_{n=1}^{\infty} a_n(7)^n$ diverges. However, this does not necessarily mean that the series $\sum_{n=1}^{\infty} a_n(3)^n$ diverges. In fact, the series $\sum_{n=1}^{\infty} a_n(3)^n$ may converge or diverge depending on the values of the coefficients $a_n$ . Therefore, we cannot conclude that the series diverges at $x = 10$ .

Statement D: The series converges at $x = 12$

To determine if the series converges at $x = 12$ , we need to evaluate the series $\sum_{n=1}^{\infty} a_n(12-7)^n = \sum_{n=1}^{\infty} a_n(5)^n$ . Since the series converges at $x = 9$ , we know that the series $\sum_{n=1}^{\infty} a_n(9-7)^n = \sum_{n=1}^{\infty} a_n(2)^n$ converges. However, this does not necessarily mean that the series $\sum_{n=1}^{\infty} a_n(5)^n$ converges. In fact, the series $\sum_{n=1}^{\infty} a_n(5)^n$ may converge or diverge depending on the values of the coefficients $a_n$ . Therefore, we cannot conclude that the series converges at $x = 12$ .

Conclusion

In conclusion, the only statement that must be true is Statement B: The series converges at $x = 7$ . This is because the series $\sum_{n=1}^{\infty} a_n(0)^n$ is equal to zero, and therefore converges.

References

[1] Rudin, W. (1976). Principles of Mathematical Analysis. McGraw-Hill.
[2] Apostol, T. M. (1974). Mathematical Analysis. Addison-Wesley.
[3] Spivak, M. (1965). Calculus. Publish or Perish.

Note

Introduction

In our previous article, we explored the power series $S(x) = \sum_{n=1}^{\infty} a_n(x-7)^n$ , which converges at $x = 9$ and diverges at $x = 14$ . We analyzed the properties of this series and determined which of the given statements must be true. In this article, we will provide a Q&A guide to help you better understand power series convergence and divergence.

Q: What is a power series?

A power series is a series of the form $\sum_{n=0}^{\infty} a_n(x-c)^n$ , where $a_n$ are coefficients, $x$ is the variable, and $c$ is a constant.

Q: What does it mean for a power series to converge?

A power series converges at a point $x = a$ if the series $\sum_{n=0}^{\infty} a_n(a-c)^n$ converges.

Q: What does it mean for a power series to diverge?

A power series diverges at a point $x = b$ if the series $\sum_{n=0}^{\infty} a_n(b-c)^n$ diverges.

Q: How do I determine if a power series converges or diverges?

To determine if a power series converges or diverges, you need to evaluate the series at the point in question. If the series converges, then the power series converges at that point. If the series diverges, then the power series diverges at that point.

Q: What is the radius of convergence of a power series?

The radius of convergence of a power series is the distance from the center of the series to the nearest point where the series diverges.

Q: How do I find the radius of convergence of a power series?

To find the radius of convergence of a power series, you can use the ratio test or the root test.

Q: What is the ratio test?

The ratio test is a method for determining the convergence or divergence of a series. It involves taking the ratio of consecutive terms in the series and evaluating the limit of this ratio.

Q: What is the root test?

The root test is a method for determining the convergence or divergence of a series. It involves taking the nth root of the nth term in the series and evaluating the limit of this root.

Q: Can a power series have multiple radii of convergence?

Yes, a power series can have multiple radii of convergence. This occurs when the series converges at multiple points, each with a different radius of convergence.

Q: How do I determine if a power series has multiple radii of convergence?

To determine if a power series has multiple radii of convergence, you need to evaluate the series at multiple points and determine the radius of convergence at each point.

Q: What is the relationship between the radius of convergence and the interval of convergence?

The radius of convergence is related to the interval of convergence of a power series. The interval of convergence is the set of all points where the series converges.

Q: How do I determine the interval of convergence of a power series?

To determine the interval of convergence of a power series, you need to evaluate the series at multiple points and determine the radius of convergence at each point.

Conclusion

In conclusion, power series convergence and divergence are complex topics that require a deep understanding of mathematical analysis. By following the Q&A guide above, you should be able to better understand the concepts of power series convergence and divergence and how to apply them to real-world problems.

References

[1] Rudin, W. (1976). Principles of Mathematical Analysis. McGraw-Hill.
[2] Apostol, T. M. (1974). Mathematical Analysis. Addison-Wesley.
[3] Spivak, M. (1965). Calculus. Publish or Perish.

Note

The above Q&A guide is based on the assumption that the power series $S(x) = \sum_{n=1}^{\infty} a_n(x-7)^n$ converges at $x = 9$ and diverges at $x = 14$ . If this assumption is not valid, then the conclusions drawn above may not be true.