# Real Analysis - Radius of Convergence

1. ### steelphantom

159
1. The problem statement, all variables and given/known data
Suppose that $$\sum$$anxn has finite radius of convergence R and that an >= 0 for all n. Show that if the series converges at R, then it also converges at -R.

2. Relevant equations

3. The attempt at a solution
Since the series converges at R, then I know that $$\sum$$anRn = M.

At -R, the series is the following: $$\sum$$an(-R)n = $$\sum$$(-1)nanRn.

I'm not sure where to go from here. I thought I needed to use the alternating series test, but how can I know that a1 >= a2 >= ... >= an for all n? Do I know this because the series converges? Thanks for your help.

2. ### Dick

25,914
Right. You can't use the alternating series test. How about a comparison test?

3. ### steelphantom

159
Thanks! So since $$\sum$$anRn converges, and an(-R)n <= anRn for all n, then $$\sum$$an(-R)n converges.

4. ### Dick

25,914
Sorry! That's wrong. I'm clearly asleep at the wheel. That's convergence for sequences. And this sort of argument only shows that the partial sums are bounded, not that they converge. Do you know the Dirichlet convergence test?

Last edited: Apr 29, 2008
5. ### steelphantom

159
I don't know that one. But the comparison test in my book says the following:

Let $$\sum$$an be a series where an >=0 for all n.
(i) If $$\sum$$an converges and |bn| <= an for all n, then $$\sum$$bn converges.

If I let an = anRn, this is >=0 for all n. And if I let bn = an(-R)n, then I have |bn| <= an for all n, so the series converges, right? What's wrong with this statement?

6. ### Dick

25,914
Nothing wrong with that. Unfortunately, I wasn't thinking of that comparison test. Hence the panic attack. Carry on.

7. ### steelphantom

159
Ok! Thanks once again for your help.

8. ### HallsofIvy

41,265
Staff Emeritus
What's wrong with using the comparison test is that it only applies to positive series. Certainly -n< (1/2)n for all n, but we can't use that to conclude that $\sum -n$ converges!

The crucial point is that every an is positive. That means that $\sum a_n x^n$, for x negative is an alternating series. What is true of alternating series?

Last edited: Apr 29, 2008
9. ### steelphantom

159
If a1 >= a2 >= ... >= an >= ... >= 0 and lim(an) = 0, then the alternating series $$\sum$$(-1)nan converges. But, like I said before, do I know that a1 >= a2 >= ... >= an because the series converges at R?

10. ### Dick

25,914
All of terms a_n*R^n are positive and it's convergent. The series is absolutely convergent. Nothing can go wrong here.