So far, our study of series has examined the question of “Is the sum of these infinite terms finite?,” i.e., “Does the series converge?” We now approach series from a different perspective: as a function. Given a value of , we evaluate by finding the sum of a particular series that depends on (assuming the series converges). We start this new approach to series with a definition.
One of the conventions we adopt is that regardless of the value of . Therefore
This is a geometric series in with .
This series is centered at . Note how this series starts with . We could rewrite this series starting at with the understanding that , and hence the first term is .
We introduced power series as a type of function, where a value of is given and the sum of a series is returned. Of course, not every series converges. For instance, in part 1 of Example 10.6.3, we recognized the series as a geometric series in .Theorem 10.2.7 states that this series converges only when .
The value of is important when understanding a power series, hence it is given a name in the following definition. Also, note that part 2 of Theorem 10.6.4 makes a statement about the interval , but the not the endpoints of that interval. A series may/may not converge at these endpoints.
The number given in Theorem 10.6.4 is the radius of convergence of a given series. When a series converges for only , we say the radius of convergence is 0, i.e., . When a series converges for all , we say the series has an infinite radius of convergence, i.e., .
The interval of convergence is the set of all values of for which the series converges.
To find the interval of convergence, we start by using the ratio test to find the radius of convergence . If , we know the series converges on , and it remains to check for convergence at the endpoints.
On the other hand, suppose for some that . Then, for sufficiently large ,. This means that the terms of are growing in absolute value, and therefore cannot converge to zero. This means that the series diverges, by Theorem 10.2.23.
If , then the power series converges for every by the ratio test, since .
If , then power series converges only when .
If , then is the radius of convergence: by the ratio test, the series converges when .
To determine the interval of convergence, plug the endpoints ( and ) into the power series, and test the resulting series for convergence. If the series converges, we include the endpoint. If it diverges, we exclude the endpoint.
Key Idea 10.6.7 allows us to find the radius of convergence of a series by applying the Ratio Test (or any applicable test) to the absolute value of the terms of the series. We practice this in the following example.
The Ratio Test shows us that regardless of the choice of , the series converges. Therefore the radius of convergence is , and the interval of convergence is .
We apply the Ratio Test to the series :
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The Ratio Test states a series converges if the limit of . We found the limit above to be ; therefore, the power series converges when , or when is in . Thus the radius of convergence is . To determine the interval of convergence, we need to check the endpoints of . When , we have the opposite of the Harmonic Series:
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The series diverges when . When , we have the series , which is the Alternating Harmonic Series, which converges. Therefore the interval of convergence is .
We apply the Ratio Test to the series :
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According to the Ratio Test, the series converges when . The series is centered at 3, and must be within of 3 in order for the series to converge. Therefore the radius of convergence is , and we know that the series converges absolutely for all in . We check for convergence at the endpoints to find the interval of convergence. When , we have:
,
which diverges. A similar process shows that the series also diverges at . Therefore the interval of convergence is .
We apply the Ratio Test to :
for all , except .
The Ratio Test shows that the series diverges for all except . Therefore the radius of convergence is .
where the domain of is a subset of the interval of convergence of the power series. One can apply calculus techniques to such functions; in particular, we can find derivatives and antiderivatives.
The theorem states that differentiation and integration do not change the radius of convergence. It does not state anything about the interval of convergence. They are not always the same.
Notice how the summation for starts with . This is because the constant term of becomes through differentiation.
Differentiation and integration are simply calculated term-by-term using the Power Rules.
Let . Find and , along with their respective intervals of convergence.
Solution1.
We find the derivative and indefinite integral of , following Theorem 10.6.9.
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In Example 10.6.3, we recognized that is a geometric series in . We know that such a geometric series converges when ; that is, the interval of convergence is . To determine the interval of convergence of , we consider the endpoints of :
which diverges.
which diverges.
Therefore, the interval of convergence of is .
To find the interval of convergence of , we again consider the endpoints of :
The value of is irrelevant; notice that the rest of the series is an Alternating Series that whose terms converge to 0. By the Alternating Series Test, this series converges. (In fact, we can recognize that the terms of the series after are the opposite of the Alternating Harmonic Series. We can thus say that .)
Notice that this summation is the Harmonic Series, which diverges. Since converges for and diverges for , the interval of convergence of is .
The previous example showed how to take the derivative and indefinite integral of a power series without motivation for why we care about such operations. We may care for the sheer mathematical enjoyment “that we can”, which is motivation enough for many. However, we would be remiss to not recognize that we can learn a great deal from taking derivatives and indefinite integrals.
Important: We stated in Key Idea 10.2.20 (in Section 10.2) that the Alternating Harmonic Series converges to , and referred to this fact again in Example 10.5.6 of Section 10.5. However, we never gave an argument for why this was the case. The work above finally shows how we conclude that the Alternating Harmonic Series converges to .
Let . Find and , and use these to analyze the behavior of .
Solution1.
We start by making two notes: first, in Example 10.6.8, we found the interval of convergence of this power series is . Second, we will find it useful later to have a few terms of the series written out:
(10.6.3)
We now find the derivative:
Since the series starts at and each term refers to , we can re-index the series starting with :
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We found the derivative of is . The only functions for which this is true are of the form for some constant . As (see Equation (10.6.3)), must be 1. Therefore we conclude that
for all .
We can also find :
We write out a few terms of this last series:
The integral of differs from only by a constant, again indicating that .
Example 10.6.12 and the work following Example 10.6.11 established relationships between a power series function and “regular” functions that we have dealt with in the past. In general, given a power series function, it is difficult (if not impossible) to express the function in terms of elementary functions. We chose examples where things worked out nicely.
Give the first 4 terms of the power series solution to , where .
Solution.
The differential equation describes a function where the derivative of is twice and . This is a rather simple differential equation; with a bit of thought one should realize that if , then , and hence . By letting we satisfy the initial condition of .
Let’s ignore the fact that we already know the solution and find a power series function that satisfies the equation. The solution we seek will have the form
for unknown coefficients . We can find using Theorem 10.6.9:
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Since , we have
The coefficients of like powers of must be equal, so we find that
etc.
The initial condition indicates that ; with this, we can find the values of the other coefficients:
and and and and .
Thus the first 5 terms of the power series solution to the differential equation is
In Section 10.7, as we study Taylor Series, we will learn how to recognize this series as describing .
Our last example illustrates that it can be difficult to recognize an elementary function by its power series expansion. It is far easier to start with a known function, expressed in terms of elementary functions, and represent it as a power series function. One may wonder why we would bother doing so, as the latter function probably seems more complicated. In the next two sections, we show both how to do this and why such a process can be beneficial.