Find The Interval Of Convergence Of

Finding the interval of convergence of a power series is a fundamental concept in calculus and real analysis. It allows us to determine the set of x-values for which a given power series converges to a finite value. Understanding this interval is crucial for applying power series in approximating functions, solving differential equations, and various other mathematical applications.

What is a Power Series?

A power series is an infinite series of the form:

$\sum_{n=0}^{\infty} c_n (x - a)^n = c_0 + c_1(x - a) + c_2(x - a)^2 + c_3(x - a)^3 + \dots$

where:

• x is a variable.
• $c_n$ are the coefficients of the series (constants).
• a is a constant called the center of the series.

The center of the power series is the value around which the series is expanded. The key question is: for what values of x does this series converge? The answer lies in finding the interval of convergence.

Convergence and Divergence

Before diving into the methods, let's clarify what convergence and divergence mean for an infinite series:

• Convergence: A series $\sum_{n=0}^{\infty} a_n$ converges if the sequence of its partial sums approaches a finite limit. In other words, as you add more terms, the sum gets closer and closer to a specific number.
• Divergence: A series diverges if the sequence of its partial sums does not approach a finite limit. The sum either grows without bound (to infinity) or oscillates without settling on a specific value.

The Interval of Convergence

The interval of convergence is the set of all x-values for which the power series converges. It always takes one of the following forms:

1. A single point: The series converges only when x = a. This is a trivial case.
2. A finite interval: The series converges for x in the interval (a - R, a + R), [a - R, a + R], (a - R, a + R], or [a - R, a + R), where R is the radius of convergence.
3. The entire real line: The series converges for all real numbers, i.e., $(-\infty, \infty)$.

Here, R is called the radius of convergence.

• If R = 0, the interval of convergence is just the single point {a}.
• If R = $\infty$, the interval of convergence is $(-\infty, \infty)$.
• If R is a positive real number, we need to test the endpoints x = a - R and x = a + R to determine whether they are included in the interval of convergence.

Finding the Radius of Convergence (R)

The most common methods for determining the radius of convergence are the Ratio Test and the Root Test.

1. Ratio Test:

The Ratio Test is generally easier to apply when the terms of the series involve factorials or exponential functions. It involves taking the limit of the ratio of consecutive terms.

Let $a_n = c_n (x - a)^n$. Then, calculate the limit:

$L = \lim_{n \to \infty} \left| \frac{a_{n+1}}{a_n} \right| = \lim_{n \to \infty} \left| \frac{c_{n+1}(x-a)^{n+1}}{c_n(x-a)^n} \right| = \lim_{n \to \infty} \left| \frac{c_{n+1}}{c_n} (x-a) \right| = |x-a| \lim_{n \to \infty} \left| \frac{c_{n+1}}{c_n} \right|$

• If L < 1, the series converges absolutely.
• If L > 1, the series diverges.
• If L = 1, the test is inconclusive.

To find the radius of convergence R, we want L < 1. Therefore:

$|x-a| \lim_{n \to \infty} \left| \frac{c_{n+1}}{c_n} \right| < 1$

$|x-a| < \frac{1}{\lim_{n \to \infty} \left| \frac{c_{n+1}}{c_n} \right|}$

Thus, the radius of convergence is:

$R = \frac{1}{\lim_{n \to \infty} \left| \frac{c_{n+1}}{c_n} \right|}$

If $\lim_{n \to \infty} \left| \frac{c_{n+1}}{c_n} \right| = 0$, then R = $\infty$. If $\lim_{n \to \infty} \left| \frac{c_{n+1}}{c_n} \right| = \infty$, then R = 0.

2. Root Test:

The Root Test is often more useful when the terms of the series involve n-th powers.

Let $a_n = c_n (x - a)^n$. Then, calculate the limit:

$L = \lim_{n \to \infty} \sqrt[n]{|a_n|} = \lim_{n \to \infty} \sqrt[n]{|c_n (x-a)^n|} = \lim_{n \to \infty} |x-a| \sqrt[n]{|c_n|} = |x-a| \lim_{n \to \infty} \sqrt[n]{|c_n|}$

• If L < 1, the series converges absolutely.
• If L > 1, the series diverges.
• If L = 1, the test is inconclusive.

To find the radius of convergence R, we want L < 1. Therefore:

$|x-a| \lim_{n \to \infty} \sqrt[n]{|c_n|} < 1$

$|x-a| < \frac{1}{\lim_{n \to \infty} \sqrt[n]{|c_n|}}$

Thus, the radius of convergence is:

$R = \frac{1}{\lim_{n \to \infty} \sqrt[n]{|c_n|}}$

If $\lim_{n \to \infty} \sqrt[n]{|c_n|} = 0$, then R = $\infty$. If $\lim_{n \to \infty} \sqrt[n]{|c_n|} = \infty$, then R = 0.

Determining the Interval of Convergence (Including Endpoints)

Once you've found the radius of convergence R, you know the series converges for a - R < x < a + R. However, you still need to determine whether the series converges at the endpoints x = a - R and x = a + R.

Steps:

1. Substitute x = a - R into the original power series. This will result in a series of constants. Apply a convergence test (e.g., Integral Test, Comparison Test, Alternating Series Test) to determine if this series converges or diverges.
2. Substitute x = a + R into the original power series. Again, this results in a series of constants. Apply a convergence test to determine if this series converges or diverges.
3. Construct the interval of convergence. Based on the convergence/divergence at the endpoints, the interval of convergence will be one of the following:
   • (a - R, a + R) - Series diverges at both endpoints.
   • [a - R, a + R) - Series converges at x = a - R but diverges at x = a + R.
   • (a - R, a + R] - Series diverges at x = a - R but converges at x = a + R.
   • [a - R, a + R] - Series converges at both endpoints.

Examples

Let's work through some examples to illustrate the process.

Example 1:

Find the interval of convergence of the power series:

$\sum_{n=0}^{\infty} \frac{x^n}{n!}$

Here, a = 0 and $c_n = \frac{1}{n!}$.

Using the Ratio Test:

$L = \lim_{n \to \infty} \left| \frac{a_{n+1}}{a_n} \right| = \lim_{n \to \infty} \left| \frac{\frac{x^{n+1}}{(n+1)!}}{\frac{x^n}{n!}} \right| = \lim_{n \to \infty} \left| \frac{x^{n+1}n!}{x^n(n+1)!} \right| = \lim_{n \to \infty} \left| \frac{x}{n+1} \right| = |x| \lim_{n \to \infty} \frac{1}{n+1} = |x| \cdot 0 = 0$

Since L = 0 < 1 for all x, the series converges for all x. Therefore, the radius of convergence R = $\infty$, and the interval of convergence is $(-\infty, \infty)$.

Example 2:

Find the interval of convergence of the power series:

$\sum_{n=1}^{\infty} \frac{(x-2)^n}{n}$

Here, a = 2 and $c_n = \frac{1}{n}$.

Using the Ratio Test:

$L = \lim_{n \to \infty} \left| \frac{a_{n+1}}{a_n} \right| = \lim_{n \to \infty} \left| \frac{\frac{(x-2)^{n+1}}{n+1}}{\frac{(x-2)^n}{n}} \right| = \lim_{n \to \infty} \left| \frac{(x-2)^{n+1}n}{(x-2)^n(n+1)} \right| = \lim_{n \to \infty} \left| \frac{n(x-2)}{n+1} \right| = |x-2| \lim_{n \to \infty} \frac{n}{n+1} = |x-2| \cdot 1 = |x-2|$

For convergence, we need L < 1, so |x - 2| < 1. This means -1 < x - 2 < 1, or 1 < x < 3. Thus, the radius of convergence R = 1.

Now, we need to test the endpoints:

• x = 1: The series becomes $\sum_{n=1}^{\infty} \frac{(1-2)^n}{n} = \sum_{n=1}^{\infty} \frac{(-1)^n}{n}$. This is the alternating harmonic series, which converges by the Alternating Series Test.
• x = 3: The series becomes $\sum_{n=1}^{\infty} \frac{(3-2)^n}{n} = \sum_{n=1}^{\infty} \frac{1}{n}$. This is the harmonic series, which diverges.

Therefore, the interval of convergence is [1, 3).

Example 3:

Find the interval of convergence of the power series:

$\sum_{n=0}^{\infty} n! (x+1)^n$

Here, a = -1 and $c_n = n!$.

Using the Ratio Test:

$L = \lim_{n \to \infty} \left| \frac{a_{n+1}}{a_n} \right| = \lim_{n \to \infty} \left| \frac{(n+1)! (x+1)^{n+1}}{n! (x+1)^n} \right| = \lim_{n \to \infty} |(n+1)(x+1)| = |x+1| \lim_{n \to \infty} (n+1)$

If x ≠ -1, then $\lim_{n \to \infty} |(n+1)(x+1)| = \infty$. Therefore, the series only converges when x = -1. The radius of convergence R = 0, and the interval of convergence is {-1}.

Example 4:

Find the interval of convergence of the power series:

$\sum_{n=1}^{\infty} \frac{(x-5)^n}{n^2 4^n}$

Here, a = 5 and $c_n = \frac{1}{n^2 4^n}$.

Using the Root Test:

$L = \lim_{n \to \infty} \sqrt[n]{\left| \frac{(x-5)^n}{n^2 4^n} \right|} = \lim_{n \to \infty} \frac{|x-5|}{\sqrt[n]{n^2} \cdot 4} = \frac{|x-5|}{4} \lim_{n \to \infty} \frac{1}{(\sqrt[n]{n})^2} = \frac{|x-5|}{4} \cdot \frac{1}{1^2} = \frac{|x-5|}{4}$

For convergence, we need L < 1, so $\frac{|x-5|}{4} < 1$. This means |x - 5| < 4, or -4 < x - 5 < 4, or 1 < x < 9. Thus, the radius of convergence R = 4.

Now, we test the endpoints:

• x = 1: The series becomes $\sum_{n=1}^{\infty} \frac{(1-5)^n}{n^2 4^n} = \sum_{n=1}^{\infty} \frac{(-4)^n}{n^2 4^n} = \sum_{n=1}^{\infty} \frac{(-1)^n}{n^2}$. This series converges absolutely because $\sum_{n=1}^{\infty} \frac{1}{n^2}$ converges (p-series with p=2 > 1).
• x = 9: The series becomes $\sum_{n=1}^{\infty} \frac{(9-5)^n}{n^2 4^n} = \sum_{n=1}^{\infty} \frac{4^n}{n^2 4^n} = \sum_{n=1}^{\infty} \frac{1}{n^2}$. This is a p-series with p = 2 > 1, so it converges.

Therefore, the interval of convergence is [1, 9].

Key Considerations and Tips

• Choosing the Right Test: The Ratio Test is often the go-to test, but the Root Test can be more efficient when dealing with terms raised to the n-th power. Consider the structure of the series terms when deciding which test to use.
• Simplifying Limits: Be careful when evaluating the limits. Remember L'Hôpital's Rule can be helpful for indeterminate forms.
• Endpoint Testing is Crucial: Don't forget to test the endpoints after finding the radius of convergence. The convergence behavior at the endpoints determines the final form of the interval of convergence. Use appropriate convergence tests like the Alternating Series Test, Comparison Test, Limit Comparison Test, or Integral Test.
• Absolute vs. Conditional Convergence: If a series converges when the absolute value of its terms converges, it converges absolutely. If a series converges, but the absolute value of its terms diverges, it converges conditionally. The endpoint x=1 in Example 2 provides a good example of conditional convergence.
• Recognizing Common Series: Familiarize yourself with common series like the geometric series, harmonic series, and p-series, as they often appear when testing endpoints. Knowing their convergence properties can save you time.
• Be Careful with Factorials: When dealing with factorials, remember that (n+1)! = (n+1) * n! This simplification is essential in the Ratio Test.
• Watch Out for Alternating Series: The Alternating Series Test is a powerful tool for determining the convergence of alternating series at the endpoints.

Applications

Understanding the interval of convergence is vital for several applications:

• Approximating Functions: Power series provide a way to represent functions as infinite polynomials. The interval of convergence determines the range of x-values for which the power series approximation is valid. For example, Taylor and Maclaurin series are used extensively in approximating functions like sin(x), cos(x), and e^x.
• Solving Differential Equations: Many differential equations can be solved using power series methods. The solution is expressed as a power series, and the interval of convergence determines the domain over which the solution is valid.
• Evaluating Integrals: Sometimes, it's difficult or impossible to find the antiderivative of a function in terms of elementary functions. Representing the function as a power series and integrating term-by-term can provide a series representation of the integral, which can then be used to approximate the integral's value.
• Complex Analysis: The concept of convergence extends to complex numbers. The region of convergence in the complex plane is often a disk centered at a, with the radius of the disk being the radius of convergence.

Conclusion

Finding the interval of convergence is a crucial skill in calculus and analysis. Mastering the Ratio Test and Root Test, combined with careful endpoint analysis, allows you to determine the x-values for which a power series converges. This knowledge is essential for applying power series in various mathematical and scientific applications, from approximating functions to solving differential equations. Remember to practice with different examples to solidify your understanding and develop your problem-solving abilities. The ability to accurately determine the interval of convergence unlocks the full potential of power series as powerful tools in mathematical analysis.