The interval of convergence is a set of x-values on which a power series converges. In other words, it’s the interval of x-values that you can plug in to make a convergent series. It’s possible for this interval to include all of the values in a series, a limited range of x-values, or just a single x-value at the center.
Radius of Convergence
The radius of convergence is half the length of the interval of convergence. For example, let’s say you had the interval (b, c). The radius of convergence will be R = (c – b) / 2. Two extremes are possible:
- The radius of convergence can be zero, which will result in an interval of convergence with a single point, a (the interval of convergence is never empty).
- Or, for power series which is convergent for all x-values, the radius of convergence is +∞.
The anchor point a is always the center of the interval of convergence. For real numbers of x, the interval is a line segment; for complex numbers, the interval will be a circle. If the radius of convergence is R, then the interval of convergence includes the open interval:
(a – R, a + R).
For example, if the radius of convergence is 1/N, then the interval of convergence includes (a – 1/N, a + 1/N); In order to find the interval of convergence, find the radius of convergence first then insert the result into (a – R, a + R).
How to Find the Radius of Convergence
Example question: What is the interval of convergence for the series:
Solution (perform the root test):
Step 1: Plug the series into the formula for the root test:
Step 2: Set the limit as an equality less than 1 (for convergence):
Step 3: Solve for x:
The Radius of Convergence is 1 (from the right side of the inequality).
Step 4: Plug your Step 3 answer for R into the interval of convergence formula:
(a – R, a + R) = (5 – 1, 5 + 1) = (4, 6).
*For a power series, the center is defined in the terms. Look for part of a general term in the series that looks like x – a. The center is “a“.
Ratio Test General Steps
The formula for the test is:
The basic steps for using the ratio test to find the radius of convergence:
Step 1: Form a ratio of an + 1/an, then simplify.
Step 2: Take the absolute value of the ratio and the limit as n → ∞.
Step 3: Use the table below to find R.
|Result from Step 2:||R|
|Zero||Infinite (i.e. convergence for all x-values)|
|N · |x – a| (N is a finite, positive number)||R = 1/N|
|Infinity||Zero (i.e. convergence at only x = a)|
A Note About Endpoints
The steps above show you how to find the interval of convergence, but they don’t tell you if the endpoints of the series are inside that interval. To find that out, plug the endpoints into the power series one at a time. Then use a convergence test to figure out if the