# Simplifying to a Geometric Series

1. Nov 30, 2011

### xago

1. The problem statement, all variables and given/known data

I have a question with asks to solve a differential equation via power series and I've done everything up to finding the recurrence relation which is a$_{n+2} = -$$\frac{a_{n}}{n+2}$
Given the initial conditions a$_{o}$ = 1 and a$_{1}$ = 0 I'm trying to simplify the series into a geometric series. The series is 1,-1/2, 1/8, -1/48, 1/480, -1/5760 etc...

3. The attempt at a solution

So far I've gotten (-1)$^{n}$ in the numerator to account for the alternating negative sign, however I can't find the denominator for the life of me...
my best attempt is $\frac{(-1)^{n}}{(2^n)}$ but it only works for the first 2 terms :S

2. Nov 30, 2011

### Mute

Don't just try to randomly guess what the pattern is by looking at the first few terms. You want to try to do it a little more systematically than that. Try to write $a_{n+2}$ in terms of $a_0$ (hopefully you noticed that all the odd coefficients are zero).

i.e., start writing
$$a_{2k+2} = -\frac{a_{2k}}{2k+2} = (-1)^2\frac{a_{2k-2}}{(2k+2)(2k)} = (-1)^3\frac{a_{2k-4}}{(2k+2)(2k)(2k-2)} = \dots$$
and then see if you can guess a pattern that gives $a_{2k+2} = a_0/(\mbox{stuff})$. (note that I set n = 2k).

Once you figure out the pattern, if you want to legitimately prove it's correct you can prove it using induction (i.e., prove it holds for k = 1, then prove that if you assume the form is true for k it follows that it's true for k+1).

Also, note that the terms in a geometric series have the form $ar^k$. Your terms won't look quite like this, so it's not exactly a geometric series.

3. Nov 30, 2011

### xago

The best I can do is 2k!! which gives 2k(2k-2)(2k-4)...right? it works up k=3 to but it's missing the 2k+2 term and doesnt work for k=4
I know 2k!! is expressed as $2^{k}$k! but how is (2k+2)!! expressed in such a way that I can calculate it?

edit*
oops, 2k!! is right I believe. I miscalculated the series in my first post, instead of -1/48, 1/480 it should have been -1/48, 1/384 etc..

Last edited: Nov 30, 2011
4. Nov 30, 2011

### obafgkmrns

Looks like 1/480 should be 1/384 instead, and 1/5760 be 1/3840. Did you skip a term?

In any case, you might want to check out Sloan's Sequences: https://oeis.org/

5. Nov 30, 2011

### Mute

If you're still not sure, just take (2k)!! and replace k with k + 1: (2(k+1))!! = (2k+2)!! = 2k+1(k+1)!, such that

$$a_{2k+2} = \frac{(-1)^{k+1}}{2^{k+1}(k+1)!}$$

6. Dec 1, 2011

### HallsofIvy

(2k+2)!!= (2(k+1))!! so it would just be $2^{k+1}(k+1)!$