# Power Series with a funky denominator

1. Oct 26, 2009

### NastyAccident

1. The problem statement, all variables and given/known data
Find the radius of convergence and interval of convergence of the following series.
$$\sum^{\infty}_{n=1}\frac{n!*x^{n}}_{5*11*17*\cdots*(6n-1)}$$

2. Relevant equations
Knowledge of Power Series.
Factorials?

3. The attempt at a solution
I'm really unsure where to start, however, I did attempt [see the attached pdf] solving the series by just attacking it head on with the ratio test. After I obtained the convergence interval, I reran the ratio test using x=6 and received an inconclusive result (L=1).

The only type of Power Series that I've dealt with similarly was:

$$\sum^{\infty}_{n=1}\frac{n^{2}x^{n}}_{2*4*6*\cdots*(2n)}$$

$$2*4*6*\cdots*(2n) = (2*1)*(2*2)*(2*3)*\cdots*(2*n) = 2^{n}(1*2*3*\cdots*n)=2^n*n!$$

However, with this particular series, as I said earlier, I'm not really sure how to handle the series because of the type of denominator I'm dealing with... Heck, I'm not even sure what this type of denominator is called.

ANY, and ALL help will be appreciated and thanked!

Sincerely,

NastyAccident.

#### Attached Files:

• ###### SumWithAFunkyDenominatorConvergence.pdf
File size:
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2. Oct 26, 2009

### Staff: Mentor

Try the ratio test. That will probably show that this series converges, and the ratio can be used to find the values of x for which it converges. That gives you your interval of convergence and the radius of convergence. You'll probably need to check the endpoints of the interval separately.

3. Oct 26, 2009

### NastyAccident

So far I've done all of that. (Please see the attached PDF. Sorry that I couldn't save it as a GIF/JPG since I currently am using MathType-Lite for Mac so I had to export my pages document containing the MathType-lite equation to a PDF.)

*Shown in PDF on original post*
R = 6
I = -6<x<6

I am having trouble with actually checking the endpoints determined since I'm unsure what the denominator actually is.

For instance, I know that if x = -6 it is an alternating series.
Thus, I should run the Alternating Series Test (AST). However, since I'm unsure what the actual denominator is. This causes me to not be able to justify the limit (part II of the AST). Justifying an+1 < an (Part I of the AST) is easy assuming that I times the denominator by (6n+1) so it becomes

5*11*17*....*(6n+1)*(6n-1).

With x = 6, I'm not even sure what test to begin with. Since I can't justify the limit... [Normally, I run the Test of Divergence first.]

Sincerely,

NastyAccident

4. Oct 26, 2009

### Staff: Mentor

Your attachment is still pending approval, so my previous post applied to what I could see in your post.

For x = -6, the general term in your series is
$$\frac{n! (-6)^n}{5 * 11 * 17 * ... * (6n - 1)}$$

This can be rewritten as
$$\frac{6*1}{5}~\frac{6*2}{11}~\frac{6*3}{17}...~\frac{6n - 6}{6n - 7}~\frac{6n}{6n - 1}$$

I have omitted the (-1)^n factor, since we already know this is an alternating series, and the goals are to determine whether the general term an in this series approaches 0 and an + 1 < an.

Pretty clearly the series is NOT monotone decreasing. To get an + 1, we multiply an by 6(n + 1)/(6n + 5), a number that is larger than 1. From this, we see that the sequence of partial sums does not approach zero, so the series diverges.

The same is true when x = 6: the series diverges at this endpoint of the convergence interval.

Last edited: Oct 26, 2009
5. Oct 26, 2009

### NastyAccident

Ah, my apologies in regards to uploading a PDF I wasn't aware of the approval requirement. Thank you for help so far as well!

Now, you converted the numerator to a form similar to the denominator and then evaluated it.

$$\frac{6*1}{5}~\frac{6*2}{11}~\frac{6*3}{17}...~\frac{6n - 6}{6n - 7}~\frac{6n}{6n - 1}$$

$$\frac{6}{5}~\frac{12}{11}~\frac{18}{17}...~\frac{6n - 6}{6n - 7}~\frac{6n}{6n - 1}$$

This makes sense so far in my mind.

To get an+1 you took the last term $$\frac{6n}{6n - 1}$$ and plugged in for n = n+1. So, an+1=$$....*\frac{6(n+1)}{6(n+1) - 1}=.....*\frac{6n+6)}{6n+6) - 1}=.....*\frac{6n+6)}{6n+5}$$

Then you were able to see that the limit as n->infinity = 1 (not zero), therefore it is not convergent by the Test of Divergence for both x=-6,6. Correct?

Sincerely,

NastyAccident

6. Oct 26, 2009

### Staff: Mentor

Yes. What you have for an is the product of n factors, each of which is slightly larger than 1, hence lim an is not zero.

7. Oct 26, 2009

### NastyAccident

Thanks! I appreciate it =)

Sincerely,

NastyAccident