Find a Fourier Series representation

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Discussion Overview

The discussion revolves around finding a Fourier series representation for the function \( f(x) = \cos(\alpha x) \) with a period of \( P = 2\pi \), where \( \alpha \) is a constant that is not an integer. Participants are exploring the calculation of Fourier coefficients and the implications of the parameter \( \alpha \) being non-integer.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant expresses difficulty in finding the Fourier series representation for the given function and requests assistance.
  • Another participant suggests using standard trigonometric identities and asks the original poster to share their attempts and specific points of confusion.
  • A third participant mentions that the Fourier coefficients are defined by certain integrals and encourages the original poster to write them down.
  • The original poster reports calculating \( a_0 \), \( a_n \), and \( b_n \) coefficients, stating \( a_0 = 0 \), \( a_n = 0 \) for \( n \neq \alpha \) and \( a_n = 1 \) for \( n = \alpha \), and \( b_n = 0 \).
  • Another participant clarifies that the parameter \( \alpha \) should be referred to as such to avoid confusion with the Fourier coefficients and questions the validity of the coefficients for non-integer \( \alpha \).
  • This participant notes that for integer values of \( \alpha \), the coefficients can be determined by inspection, suggesting that the problem is less interesting in that case.
  • The original poster acknowledges the non-integer nature of \( \alpha \) but expresses difficulty in writing down the series coefficients for non-integer values, indicating a perceived complexity in finding a general expression.

Areas of Agreement / Disagreement

Participants have not reached a consensus on how to approach the Fourier series representation for non-integer \( \alpha \). There are differing views on the necessity of performing Fourier integrals versus using inspection for integer values of \( \alpha \).

Contextual Notes

The discussion highlights the challenges in deriving a general expression for the Fourier coefficients when \( \alpha \) is not an integer, and the potential need for limits in evaluating cases where \( \alpha \) is an integer.

math_trouble
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I'm having problem finding the representation for the Fourier series with

function f of period P = 2*pi such that f (x) = cosαx, −pi ≤ x ≤ pi , and α ≠ 0,±1,±2,±3,K is a
constant.

Any help is appreciated...
 
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Hi math_trouble! :wink:

You probably need one of the standard trigonometric identities for cosAcosB.

Anyway, show us what you've tried, and where you're stuck, and then we'll know how to help! :smile:
 
The Fourier coefficients are certain integrals. Write them down. If you can't do them, let us know.
 
i evaluated a0,an and bn term
i get a0=0

an= 0 when a not equals to n & 1 when a equals to n

bn= 0

i know the general Fourier series representation is in :
a0/2 + SUM(ancos(nx) + bnsin(nx))

but then I am stuck on how to apply the general term to this case
 
Hi Math_Trouble. Just to save confusion, can we call the parameter \alpha instead of "a", so as not to confuse it with the a_n Fourier coefficients.

Your a_n and b_n are correct for integer values of the parameter \alpha, but I thought that you wanted an expression that is valid for non-integer alpha. I'm a little confused here because you say that you've "evaluated the terms" which implies that you have an expression to evaluate. If so then where is your expression and is it valid for non-integer \alpha?

The fact is that for integer values of alpha you don't even need to do the Fourier integrals to determine the series coefficients. You can do it "by inspection" since the waveform is already a perfect cosine wave. It's not that the Fourier integrals don't work for integer \alpha, they do, it's just that the problem is not really interesting for that case (which is why I presume that they explicitly called for a non integer alpha in the question).

If you just do the Fourier integrals then you should get an expression that is valid for real (integer and non integer) values of the parameter (though you may need to take limits to evaluate the integer cases). Show us your working so far and we can help you.
 
Last edited:
Hi uart..thx for reminding tat \alpha is not an integer:smile:

but now I am having trouble again to write down the series coefficient for \alpha \neq integer because it seems to be too many values and not like a general expression could express them all
 

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