Developping sin(x) on [0, pi] as a serie of cosines

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Discussion Overview

The discussion revolves around the possibility of developing the sine function, sin(x), on the interval [0, pi] as a series of cosines, particularly in the context of extending the function to be even and maintaining a period of 2pi.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant questions the feasibility of developing sin(x) as a cosine series while requiring the extended even function to have a period of 2pi.
  • Another participant suggests extending sin(x) to -sin(x) for x < 0 to create an even function, proposing that a cosine series can then be applied on the interval [-pi, pi], which would also be valid on [0, pi].
  • A third participant points out that the period of the function |sin(x)| is actually pi, which contradicts the requirement for a period of 2pi.
  • A later reply acknowledges the correctness of the third participant's claim.

Areas of Agreement / Disagreement

Participants express differing views on the possibility of developing sin(x) as a cosine series under the specified conditions, indicating that the discussion remains unresolved.

Contextual Notes

The discussion highlights potential limitations regarding the assumptions about function periodicity and the implications of extending sin(x) to an even function.

quasar987
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Am I crazy or this is impossible to do if we require that the period of the extended even function be 2pi ?

Yet, this question comes in my textbook before the section concerning Fourier extension of functions of period other than 2pi. So there must be a way?
 
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I'm pretty sure it's possible. Extend sin(x) before x=0 as -sin(x) so the function is even. Now do a cosine series on the interval [-pi,pi] and it will be valid on [0,pi] and have period 2pi.
 
This is what I did, but the period of this function (|sin(x)|) is pi.
 
Arr, you're right.
 

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