Proving T_n(x) as Polynomials: Trig Homework Solution

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Homework Help Overview

The discussion revolves around proving that the Chebyshev polynomials, defined as T_n(x) = cos(n arccos(x)), are polynomials of degree n. The original poster questions the validity of this for various values of n, particularly noting discrepancies when n=2 and n=1.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants discuss using induction as a potential method to demonstrate that cos(ny) is an nth degree polynomial in cos(y). There are also questions about the validity of the polynomial form for specific values of n, particularly n=1 and n=2.

Discussion Status

There is an ongoing exploration of the properties of the Chebyshev polynomials, with participants questioning the assumptions made regarding the polynomial nature for different values of n. Some guidance has been suggested regarding the use of induction, but no consensus has been reached on the validity of the polynomial form.

Contextual Notes

Participants note that the function f(x) = x is a polynomial for n=1, raising questions about the original poster's assertion that it fails for this case. There is also an acknowledgment that the discussion may be overlooking certain aspects of the problem.

Dragonfall
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How do I show that

[tex]T_n(x)=\cos(n\arccos(x))[/tex]

are actually polynomials of degree n?

If n=2 then

[tex]T_2(x)=x^2-(1-x^2)[/tex]

but this seems to break down as n increases.
 
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So if x=cos(y), you want to show cos(ny) is a nth degree polynomial in cos(y). Induction is usually the first thing to try whenever there's an n knocking about.
 
It also definitely fails for n=1, but since you started with 2, maybe we're ignoring that
 
Office_Shredder said:
It also definitely fails for n=1, but since you started with 2, maybe we're ignoring that

How does it fail for n=1? The function f(x)=x is a polynomial.
 

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