Expansion of cos 15 x interms of cos

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Discussion Overview

The discussion centers around the expansion of cos(15x) in terms of cos(theta), exploring its mathematical representation and connections to Chebyshev polynomials. Participants share their findings and insights related to this expansion, including verification methods and relationships to polynomial functions.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents the expansion of cos(15x) as a polynomial in cos(theta) and claims to have verified it using symbolic MATLAB.
  • Another participant expresses appreciation for the shared expansion, indicating its usefulness.
  • A participant notes the similarity between the expansion and the Chebyshev polynomial of the 15th degree, suggesting a connection between the two.
  • Some participants challenge the assertion that Chebyshev functions are merely cos functions, emphasizing that while T_n(cos(theta)) = cos(n*theta), the Chebyshev polynomial itself is a polynomial, not a cosine function.
  • A later reply retracts a previous statement regarding the relationship between Chebyshev functions and cosine functions, acknowledging the inaccuracy of the earlier claim.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between Chebyshev polynomials and cosine functions, with some asserting a connection while others challenge this notion. The discussion remains unresolved regarding the interpretation of these mathematical relationships.

Contextual Notes

Some statements made in the discussion contain assumptions about the nature of Chebyshev polynomials and their relationship to cosine functions that remain unexamined. The mathematical steps leading to the expansion are not fully detailed, and the implications of the polynomial representation are debated.

somani.utsav
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I have expanded cos 15x interms of cos. I hope some one who needs it can use it. I have verified this formulae using symbolic matlab: so its geniune

cos 15*theta = 16384*cos(theta)^15 - 61440*cos(theta)^13 + 92160*cos(theta)^11 - 70400*cos(theta)^9 + 28800*cos(theta)^7 - 6048*cos(theta)^5 + 560*cos(theta)^3 - 15*cos(theta)
 
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Thanks, I've been looking everywhere for this!
 
One might notice a similarity with the Chebyshev polynomial of 15th degree:

T15(x)=16384x15-61440x13+92160x11-70400x9+28800x7-6048x5+560x3-15x
 
This means that chebyshev function is nothing but a cos function
 
somani.utsav said:
This means that chebyshev function is nothing but a cos function

That's not quite accurate, but [itex]T_n(\cos \theta) = \cos(n \theta)[/itex].
 
somani.utsav said:
This means that chebyshev function is nothing but a cos function
Does [itex]16384 x^{15}-61440 x^{13}+92160 x^{11}-70400 x^9+28800 x^7-6048 x^5+560 x^3-15 x[/itex] look like "a cos function"? It's a polynomial. Like the other Chebychev polynomials, it is a rather useful polynomial.
 
AlephZero said:
That's not quite accurate, but [itex]T_n(\cos \theta) = \cos(n \theta)[/itex].

I retract my previous statement
 

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