Finding convergence of a recursive sequence

muzak
Messages
42
Reaction score
0

Homework Statement


x_{n+1} = (x_{n} + 2)/(x_{n}+3), x_{0}= 3/4

Homework Equations


The Attempt at a Solution


I've worked out a few of the numbers and got 3/4, 11/15, 41/56, 153/209, ...
It seems to be monotone and bounded below indicating it does converge I think. I need help figuring out what it converges to if it does. I've never really done convergence on a recursive sequence.
 
Physics news on Phys.org
Once you know it converges, it is easy to find the limit. Suppose ##x_n\to a## and take the limit of both sides of the recursion to figure out ##a##. Note this argument does not show it converges, only that if it does, the limit is ##a##. You still have to show it is monotone and bounded (if it is).
 
Last edited:
what is the first thing you think of when you see "n+1"?
 
Last edited:
If the sequence converges, to x, say, then, taking the limit on both sides, we must have
x= \frac{x+ 2}{x+ 3}
Solve that for x.

Of course, it is not enough to say "It seems to be monotone and bounded below", you must show that it is monotone, you have to show it is.
 
Attached.
 

Attachments

  • 001.jpg
    001.jpg
    19.3 KB · Views: 514
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...

Similar threads

Replies
2
Views
1K
Replies
4
Views
1K
Replies
17
Views
977
Replies
6
Views
2K
Replies
9
Views
2K
Replies
5
Views
2K
Replies
3
Views
1K
Back
Top