# Another series question

1. May 5, 2008

### daniel_i_l

1. The problem statement, all variables and given/known data
Prove that if $$0 < \alpha < 1$$ and
$$\sqrt[n]{|a_n|} <= 1 - \frac{1}{n^{\alpha}}$$ for all n >= 1 then the series
$$\sum a_n$$ converges.

2. Relevant equations

3. The attempt at a solution

First I did:
$$|a_n| <= \left({1 - \frac{1}{n^{\alpha}}}\right)^n = \left({{1 - \frac{1}{n^{\alpha}}}^{n^{\alpha}}}\right)^{n^{1-\alpha}}$$
and since the limit at infinity of $$\left({{1-\frac{1}{n^{\alpha}}}^{n^{\alpha}}}\right)$$ is
$$\frac{1}{e}$$ then there exists an N so that for all n>N
$$|a_n| <= \left({\frac{2}{e}}\right)^{n^{1-\alpha}}$$
But how can I show that $$\sum \left({\frac{2}{e}}\right)^{n^{1-\alpha}}$$
converges?
Is there a better way to do this?
Thanks.

Last edited: May 5, 2008
2. May 5, 2008

### e(ho0n3

How about using the Root Convergence Test?

3. May 5, 2008

### daniel_i_l

For the Root Convergence Test you need to have a q<1 so that for all n
$$\sqrt[n]{|a_n|} <=q$$ but since
$$1 - \frac{1}{n^{\alpha}}$$
goes to 1 it doesn't apply in this case.

4. May 6, 2008

### daniel_i_l

Is there another way to approach the problem?
Thanks.

5. May 6, 2008

### e(ho0n3

$$\left({\frac{2}{e}}\right)^{n^{1-\alpha}} = r^n$$

where

$$r = \left({\frac{2}{e}}\right)^{1/n^\alpha}$$

If you can show that |r| < 1, then you can use the geometric series.

6. May 6, 2008

### daniel_i_l

Well, $$\frac{2}{e} < 1$$ and $$\frac{1}{n^{\alpha}} > 0$$ for all n
and so
$$\left({\frac{2}{e}}\right)^{\frac{1}{n^{\alpha}}} < 1$$ right?
But the problem is that now r depends on n and r goes to 1 as n goes to infinity so how can you use the geometric series?

Last edited: May 6, 2008
7. May 6, 2008

### e(ho0n3

Oops. That's right. So how about using a comparison test with a geometric series?

8. May 6, 2008

### daniel_i_l

I tried that and didn't really get any where. I think that maybe
$$\sum \left({\frac{2}{e}}\right)^{n^{1-\alpha}}$$
doesn't even converge. What I have to prove is that
$$\left({1 - \frac{1}{n^{\alpha}}}\right)^n$$
converges. Do you have any ideas on how to do that?
Thanks.

9. May 6, 2008

### e(ho0n3

I made a perl script to compute the sum with alpha = 0.5. The sum converges to about 20.8.

Show us what you did with the comparison test.

10. May 6, 2008

### daniel_i_l

I started with
$$b_n =\left({\frac{2}{e}}\right)^{n^{1-\alpha}}$$ and
$$c_n = \left({\frac{2}{e}}\right)^{n}$$
I know that c_n converges so if b_n/c_n goes to some finite number that proves that b_n converges to. But:
$$\frac{b_n}{c_n} = \left({\frac{2}{e}}\right)^{n(n^{-\alpha}-1)}$$
and since $$\left({\frac{2}{e}}\right)^{n}$$ goes to 0 and
$$(n^{-\alpha}-1)$$ goes to -1 and so b_n/c_n goes to infinity.
Is there any series that's better to compare it to?
And by the way, how did you approximate the sum? Did you simply add up terms up to a "very high"(~1000000) number?
Thanks for you help

11. May 6, 2008

### e(ho0n3

Yes I did.

12. May 6, 2008

### daniel_i_l

Then how can I solve the problem if the root test and the comparison test don't work?
Thanks.

13. May 6, 2008

### e(ho0n3

I've realized that comparing to a geometric series won't work because there will always be a term greater than the corresponding term in the geometric series.

14. May 6, 2008

### daniel_i_l

So what else is there to do to solve the original problem?
Thanks.

15. May 6, 2008

### e(ho0n3

I've been looking at other convergence tests but I don't see one that would work. This one is tough.

16. May 6, 2008

### Dick

Ok, try this. Show (1-x/n)^n<=exp(-x) for 0<x<n. I think it's true. Prove it. Then if all goes well, you can compare you original series with exp(-n^p) where p=1-alpha. So 0<p<1. That also fails the root test and the ratio test, etc. That probably means we are on the right track. Series expand the exponential in 1/exp(n^p). Notice the increasing powers of n^p? At some point you reach a point where (n^p)^k=n^(p*k) is such that p*k>1. Now what??

17. May 6, 2008

### e(ho0n3

$(1 - \frac{x}{n})^n \le e^{-x}$ is equivalent to

(*) n ln(1 - x/n) ≤ -x

As $x \to 0$, (*) becomes 0 ≤ 0. As $x \to n$, (*) becomes $-\infty \le -n$. Both sides are decreasing in the interval (0, n) but the LHS seems to be decreasing faster. This is conclusively shown by comparing the derivatives of n ln(1 - x/n) with -x.

Why are you concluding that? It seems to be true when k = 1, but then pk < 1. What is the point of this?

18. May 6, 2008

### Dick

I know you are interested, but try not to take over from daniel_i_l and work on his problem for him, ok? I.e. stop showing the mechanics of something I left as a routine exercise. But if your working is correct then that would work for a proof of the first one. I didn't do it that way and I'm not going to check it. For the second part the strategy is to compare with a p-series. So I want an exponent bigger than one. Enuf said?

19. May 7, 2008

### e(ho0n3

Oops. Sorry about that. I don't want to take over someone else's thread.

20. May 25, 2008

### daniel_i_l

Do you mean this:
$$\frac{1}{1 + e^{{n^p}} + \frac{e^{{n^p}^2}}{2!} + ... +\frac{e^{{n^p}^k} }{k!}...}$$
?
I don't see how this is easier to evaluate - especially with all the factorials.

Last edited: May 25, 2008