Monotony of a recurrence relation

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    Recurrence Relation
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Discussion Overview

The discussion revolves around determining whether a specific recurrence relation is increasing or decreasing. The relation in question is defined as A1 = 1 and An = (An-1)^5 - 3. Participants explore methods to mathematically demonstrate the behavior of the sequence generated by this relation.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant asserts that the sequence decreases because terms for n ≥ 2 are negative numbers raised to an odd power, but struggles to demonstrate this mathematically.
  • Another participant questions why induction does not work for the initial claim and suggests that if A_n < 0, then A_{n+1} < A_n, which could serve as an inductive step.
  • A different participant proposes that |A_n| is strictly increasing, implying that if all terms are negative, A_n is strictly decreasing. They suggest showing that |A_{n+1}| > |A_n| as a means to support this argument.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of induction as a method for demonstrating the behavior of the recurrence relation. There is no consensus on a definitive method or conclusion regarding the monotonicity of the sequence.

Contextual Notes

Some assumptions regarding the behavior of the terms and the conditions under which the recurrence relation operates are not fully explored. The discussion does not resolve the mathematical steps necessary to demonstrate the claims made.

Who May Find This Useful

This discussion may be useful for individuals interested in recurrence relations, mathematical induction, and the analysis of sequences in a mathematical context.

Keru
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What method should i use to know if a recurrence relation is increasing or decreasing?
i was given the following relation:
A1 = 1
An=(An-1)^5 - 3

I know for sure it actually decreases since every term for n>=2 is a negative number raised to and odd number, but i don't know how to demonstrate it mathematically. I tried using induction, but it doesn't work...

Thanks to whoever can answer me.
 
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Keru said:
What method should i use to know if a recurrence relation is increasing or decreasing?
i was given the following relation:
A1 = 1
An=(An-1)^5 - 3

I know for sure it actually decreases since every term for n>=2 is a negative number raised to and odd number, but i don't know how to demonstrate it mathematically. I tried using induction, but it doesn't work...

Thanks to whoever can answer me.

Why do you think induction doesn't work?

It's clear that if ##A_n < 0## then ##A_{n+1} < A_n## and that's your inductive step.
 
Keru said:
What method should i use to know if a recurrence relation is increasing or decreasing?
i was given the following relation:
A1 = 1
An=(An-1)^5 - 3

I know for sure it actually decreases since every term for n>=2 is a negative number raised to and odd number, but i don't know how to demonstrate it mathematically. I tried using induction, but it doesn't work...

Thanks to whoever can answer me.

You can show that |A_n| is strictly increasing. If all terms are negative this shows that A_n is strictly decreasing. For n \geq 2 you have that |A_{n+1}| = |A_n^5 - 3| = |A_{n}|^5 + 3 &gt; |A_n|^5. If you can show that |A_{n}|^5 &gt; |A_n| you are done.
 
Thanks guys, very quick and useful answers! I'll keep practising so i can see it by myself next time!
 

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