Polynomials of odd degree can have no critical point.

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Discussion Overview

The discussion centers around the properties of polynomials, specifically focusing on polynomials of odd and even degrees, their roots, and the existence of critical points. Participants explore assertions regarding the existence of real roots and critical points for polynomials with real coefficients.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that there exists a polynomial of any even degree that has no real roots.
  • Others argue that polynomials of odd degree must have at least one real root, which implies that polynomials of even degree have at least one critical point.
  • Some participants challenge the assertion that polynomials of odd degree can have no critical points, providing counterexamples such as \(y = x^3 + x\) and \(y = \frac{x^3}{3} - x\), which have critical points.
  • There is confusion regarding the definition of "critical point," with some participants clarifying that it refers to points where the derivative is zero.
  • One participant expresses uncertainty about whether a polynomial of even degree should have at least one critical point, reiterating this question multiple times.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the assertions regarding critical points and roots of polynomials. Multiple competing views remain, particularly concerning the properties of odd-degree polynomials and their critical points.

Contextual Notes

Participants reference the Intermediate Value Theorem and provide examples to support their claims, but the discussion remains unresolved regarding the implications of these properties for polynomials of odd and even degrees.

batballbat
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Are these assertions true?
I am referring to polynomials with real coefficients.
1. There exists of polynomial of any even degree such that it has no real roots.
2. Polynomials of odd degree have atleast one real root
which implies that polynomial of even degree has atleast one critical point.
3. Polynomials of odd degree can have no critical point.
 
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The first two are. What do you mean by "critical point"? If you mean a place where the derivative is zero, then this is not right.

For 1) a polynomial of degree two that has no real roots is x^2 +1. Do you see how to generalise? For 2) you want to use the Intermediate Value Theorem.
 


2) not true: example y=x3 + x. It has a real root at x=0, but the derivative is never zero (for real x).
3) not true: example y=x3/3 - x. Critical points at x = 1, -1.
 


Did you edit #2 from the original post to add the second part? Because I didn't mean to include that part in my response. I was only saying that the first line of #2 is correct.
 


i don't get it.
Shouldnt a polynomial of even degree have atleast one critical point?

in question 3. i meant for any odd degree we can find a polynomial which has no critical point.
 


batballbat said:
i don't get it.
Shouldnt a polynomial of even degree have atleast one critical point?

in question 3. i meant for any odd degree we can find a polynomial which has no critical point.


All of them are true:

(1) \,(x^2+1)^n\,

(2) The intermediate value theorem for continuous functions

(3) \,\int (x^2+1)^ndx\,

DonAntonio
 


batballbat said:
i don't get it.
Shouldnt a polynomial of even degree have atleast one critical point?

in question 3. i meant for any odd degree we can find a polynomial which has no critical point.

Yes, it does, apparently I have misread your post twice now. If all odd polys. have a real root, then this means that the derivative of all even degree polys. have a zero, which means all even degree polys. have a critical point.
 

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