How Did the Author Determine the Fourier Sine Series for x^2?

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SUMMARY

The discussion centers on the determination of the Fourier sine series for the function \(x^2\). Participants clarify that the expression referenced in the book is not the half-range sine series for \(x^2\) due to the discontinuity of its odd periodic extension, which affects the convergence of coefficients. Instead, the correct sine series corresponds to the function \(f(x) = x(\pi - x)\). This distinction is crucial for accurate understanding and application of Fourier series in mathematical analysis.

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  • Understanding of Fourier series concepts
  • Familiarity with sine series expansions
  • Knowledge of function discontinuities and their effects on series convergence
  • Basic proficiency in mathematical analysis and series convergence
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  • Study the derivation of the Fourier sine series for \(x^2\)
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I'm having trouble understanding a part in my book.

second to last paragraph where it says 4.2 must be the Fourier sine series for x^2, how did the author arrive at that?


http://i.imgur.com/gLLUYXw.jpg
 
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The author was familiar enough with Fourier series to recognize the expression arrived at is the sine expansion of ##x^2##. To verify, simply find the Fourier sine series of ##x^2##.

There is no reason why most people would recognize this, especially if you're just learning it, so I wouldn't worry about it too much.
 
Something is wrong there. That is not the half range sine for series for ##x^2##. The odd periodic extension of ##x^2## is discontinuous so the coefficients could converge no faster than order of ##\frac 1 n##. It is in fact the correct sine series for ##f(x) = x(\pi -x)## as is stated.
 

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