Is there a non-constant polynomial such that....

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Discussion Overview

The discussion revolves around the existence of a non-constant polynomial \( p \) with positive coefficients such that the function \( x \mapsto p(x^2) - p(x) \) is decreasing on the interval \([1, +\infty)\). Participants explore methods to analyze the behavior of this function and the implications of polynomial properties on its monotonicity.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant questions how to determine if the function \( p(x^2) - p(x) \) is decreasing, suggesting the use of properties of increasing and decreasing functions.
  • Another participant notes that if \( p(x) \) has degree \( n \), then \( p(x^2) \) will have degree \( 2n \), prompting a discussion about the behavior of the difference \( p(x^2) - p(x) \) as \( x \) increases.
  • Some participants assert that as \( x \) becomes large, both \( p(x^2) \) and \( p(x) \) diverge to infinity, raising questions about the comparison of these divergent quantities when subtracted.
  • A later reply emphasizes the need to clarify the behavior of the difference \( p(x^2) - p(x) \) and how to determine which term grows faster as \( x \) approaches infinity.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the behavior of the function \( p(x^2) - p(x) \) and whether it can be conclusively shown to be decreasing. Multiple viewpoints on the implications of polynomial growth and divergence remain unresolved.

Contextual Notes

There are limitations regarding the assumptions made about the polynomial's degree and the behavior of its coefficients. The discussion does not resolve how to rigorously compare the divergent terms \( p(x^2) \) and \( p(x) \).

karseme
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It is required to determine if there is a non-constant polynomial p with positive coefficients such that function $x \mapsto p(x^2)-p(x)$ is decreasing on $[1,+\infty \rangle$. What should I do here? How should I exactly determine that? What is the right method? My idea was to use somehow the fact that composition of decreasing and increasing function is decreasing etc. But, then again I have a polynomial here which can be of any degree. So, I am not sure if it would be increasing on the whole domain or not, but I guess I can somehow use the fact that all coefficients are positive. What's to be done here?
 
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karseme said:
It is required to determine if there is a non-constant polynomial p with positive coefficients such that function $x \mapsto p(x^2)-p(x)$ is decreasing on $[1,+\infty \rangle$. What should I do here? How should I exactly determine that? What is the right method? My idea was to use somehow the fact that composition of decreasing and increasing function is decreasing etc. But, then again I have a polynomial here which can be of any degree. So, I am not sure if it would be increasing on the whole domain or not, but I guess I can somehow use the fact that all coefficients are positive. What's to be done here?
If the polynomial $p(x)$ has degree $n$ then $p(x^2)$ will have degree $2n$. What does that tell you about the behaviour of $p(x^2)-p(x)$ as $x$ gets large?
 
I would say that as x gets large the value of polynomial gets large and as it converges to the infinity the value converges to the infinity...
 
karseme said:
I would say that as x gets large the value of polynomial gets large and as it converges to the infinity the value converges to the infinity...
First, it can't "converge to infinity" it diverges.

Second, a detail. I know this almost doesn't need to be mentioned but you need to answer this question: p(x^2) and p(x) both diverge. What happens when you subtract the two divergent quantities: p(x^2) - p(x)? How do you show which is "bigger?"

-Dan
 

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