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8.4: Boundary Issues and Abel’s Theorem

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Learning Objectives
  • Explain the Abel's theorem

Summarizing our results, we see that any power series anxn has a radius of convergence r such that anxn converges absolutely when |x|<r and diverges when |x|>r. Furthermore, the convergence is uniform on any closed interval [b,b](r,r) which tells us that whatever the power series converges to must be a continuous function on (r,r). Lastly, if

f(x)=n=0anxn

for x(r,r), then

f(x)=n=0annxn1

for x(r,r) and

xt=0f(t)dt=n=0anxn+1n+1

for x(r,r).

Thus power series are very well behaved within their interval of convergence, and our cavalier approach from Chapter 2 is justified, EXCEPT for one issue. If you go back to Exercise Q1 of Chapter 2, you see that we used the geometric series to obtain the series,

arctanx=n=0(1)n12n+1x2n+1.

We substituted x=1 into this to obtain π4=n=0(1)n12n+1. Unfortunately, our integration was only guaranteed on a closed subinterval of the interval (1,1) where the convergence was uniform and we substituted in x=1. We “danced on the boundary” in other places as well, including when we said that

π4=1x=01x2dx=1+n=1(n1j=0(12j)n!)((1)n2n+1)

The fact is that for a power series anxn with radius of convergence r, we know what happens for x with |x|<r and x with |x|>r. We never talked about what happens for x with |x|=r. That is because there is no systematic approach to this boundary problem. For example, consider the three series

n=0xn,n=0xn+1n+1,n=0xn+2(n+1)(n+2)

They are all related in that we started with the geometric series and integrated twice, thus they all have radius of convergence equal to 1. Their behavior on the boundary, i.e., when x=±1, is another story. The first series diverges when x=±1, the third series converges when x=±1. The second series converges when x=1 and diverges when x=1.

Even with the unpredictability of a power series at the endpoints of its interval of convergence, the Weierstrass-M test does give us some hope of uniform convergence.

Exercise 8.4.1: Weierstrass-M test

Suppose the power series anxn has radius of convergence r and the series anrn converges absolutely. Then anxn converges uniformly on [r,r].

Hint

For |x|r, |anxn||anrn|.

Unfortunately, this result doesn’t apply to the integrals we mentioned as the convergence at the endpoints is not absolute. Nonetheless, the integrations we performed in Chapter 2 are still legitimate. This is due to the following theorem by Abel which extends uniform convergence to the endpoints of the interval of convergence even if the convergence at an endpoint is only conditional. Abel did not use the term uniform convergence, as it hadn’t been defined yet, but the ideas involved are his.

Theorem 8.4.1: Abel’s Theorem

Suppose the power series anxn has radius of convergence r and the series anrn converges. Then anxn converges uniformly on [0,r].

The proof of this is not intuitive, but involves a clever technique known as Abel’s Partial Summation Formula.

Lemma 8.4.1: Abel’s Partial Summation Formula

Let a1,a2,...,an,b1,b2,...,bn be real numbers and let Am=mk=1ak. Then

a1b1+a2b2+···+anbn=n1j=1Aj(bjbj+1)+Anbn

Exercise 8.4.2

Prove Lemma 8.4.1.

Hint

For j>1, aj=AjAj1.

Lemma 8.4.2: Abel’s Lemma

Let a1,a2,...,an,b1,b2,...,bn be real numbers with b1b2...bn0 and let Am=mk=1ak. Suppose |Am|B for all m. Then |nj=1ajbj|Bb1.

Exercise 8.4.3

Prove Lemma 8.4.2.

Exercise 8.4.4

Prove Theorem 8.4.1.

Hint

Let ϵ>0. Since n=0anrn converges then by the Cauchy Criterion, there exists N such that if m>n>N then |mk=n+1akrk|<ϵ2. Let 0xr. By Lemma 8.4.2,

|mk=n+1akxk|=|mk=n+1akrk(xr)k|ϵ2(xr)n+1ϵ2

Thus for 0xr, n>N,

|k=n+1akxk|=limn|mk=n+1akxk|ϵ2<ϵ

Corollary 8.4.1

Suppose the power series anxn has radius of convergence r and the series an(r)n converges. Then anxn converges uniformly on [r,0].

Exercise 8.4.5

Prove Corollary 8.4.1.

Hint

Consider an(x)n.


This page titled 8.4: Boundary Issues and Abel’s Theorem is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Eugene Boman and Robert Rogers (OpenSUNY) via source content that was edited to the style and standards of the LibreTexts platform.

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