A nonlinear recurrence relation

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Discussion Overview

The discussion revolves around a nonlinear recurrence relation of the form $$a_{n+1}a_n^2 = a_0$$ and its variations. Participants explore the nature of the recurrence, potential solutions, and the uniqueness of fixed points within the context of dynamical systems. The scope includes mathematical reasoning and conceptual clarification.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants express confusion about the recurrence relation and seek clarification on its context, questioning whether it is related to homework or a specific application.
  • One participant proposes a form of the recurrence relation, suggesting that it might oscillate between very small and very large values until reaching a division by zero or infinity.
  • Another participant emphasizes the importance of isolating $$a_{n+1}$$ to understand the recurrence better.
  • Several participants discuss the existence of fixed points, with one asserting there is exactly one fixed point at $$a_n=1$$, though it is noted as not being stable.
  • There is a discussion about the conditions under which the sequence converges, with claims that if $$a_n$$ is not equal to 1, the sequence does not converge, implying no other fixed points exist.
  • One participant mentions that the fixed point condition leads to a cubic equation with three solutions, noting that only $$a_n=1$$ results in a fixed point, while another points out that the two complex cube roots of 1 may also be considered fixed points.

Areas of Agreement / Disagreement

Participants generally agree on the existence of a fixed point at $$a_n=1$$, but there is contention regarding the stability of this point and the nature of other potential fixed points, particularly the complex solutions. The discussion remains unresolved regarding the implications of these findings.

Contextual Notes

Some participants express uncertainty about the initial conditions and the domain of $$a_n$$, which may affect the analysis of the recurrence relation and its fixed points.

Wuberdall
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Hi Physics Forums,

I am stuck on the following nonlinear recurrence relation
$$a_{n+1}a_n^2 = a_0,$$
for ##n\geq0##.

Any ideas on how to defeat this innocent looking monster?

I have re-edited the recurrence relation
 
Last edited:
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Can you provide some context here? Is this homework? In what book / course did you come across this? Is this for a particular fractal graph?

It seems that it would oscillate from very small to very large until you're dividing by zero or by infinity.

##a_{n+1} = 1 / { a_n^2 } ## where ##a_n \neq 0##
 
Wuberdall said:
Hi Physics Forums,

I am stuck on the following nonlinear recurrence relation
$$a_{n+1}a_n^2 = 1,$$
for ##n\geq0##.

Any ideas on how to defeat this innocent looking monster?
What do you want to know about it? Have you tried isolating ##a_{n+1}## on one side of the equation?
 
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Wuberdall said:
I am stuck
In what way ?
This homework ? Please use the template and provide a full problem description and an attempt at solution tohat shows where you are stuck ...
 
Hi, this is not homework or course related. I am trying to determine if a fixed point for a certain dynamical system is unique. In doing so I come across the above recurrence relation.

So what I am really looking for, is a solution and whether or not this solution is unique
 
Wuberdall said:
Hi, it is not homework or course related. I am trying to determine if a fixed point for a certain dynamical system is unique. In doing so I come across the above recurrence relation.

So what I am really looking for, is a solution and whether or nor this solution is unique
It has exactly one fixed point at ##a_n=1##, though it is not a stable fixed point.
 
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Where do the ##a_n## live and are there initial conditions?
 
tnich said:
It has exactly one fixed point at ##a_n=1##, though it is not a stable fixed point.
Thanks, this is exactly what I was looking for and also what my intuition told me.

How do you conclude that their is exactly one fixed point ?
 
Wuberdall said:
Thanks, this is exactly what I was looking for and also what my intuition told me.

How do you conclude that their is exactly one fixed point ?
If ##a_n## is not 1, then the sequence does not converge, so there can be no other fixed point.
 
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  • #10
tnich said:
If ##a_n## is not 1, then the sequence does not converge, so there can be no other fixed point.
A fixed point must satisfy ##a_{n+1}=a_n##. In this case that results in ##a_n a_n^2=1## which has three solutions (two of which are complex), but only ##a_n=1## results in a fixed point.
 
  • #11
tnich said:
If ##a_n## is not 1, then the sequence does not converge, so there can be no other fixed point.

Thanks, for your time.

I have figured it out now. It turned out that I was a bit rusty. So I found my old and dusty book by Strogatz on my bookshelf. All your comments make complete sense now and I see why they are true.

I wish you a happy and sunny weekend.
 
  • #12
tnich said:
A fixed point must satisfy ##a_{n+1}=a_n##. In this case that results in ##a_n a_n^2=1## which has three solutions (two of which are complex), but only ##a_n=1## results in a fixed point.
Oops, no I think the two complex cube roots of 1 also are fixed points.
 
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  • #13
Wuberdall said:
I wish you a happy and sunny weekend.

It's only Monday. The weekend is a long way off.
 

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