Why Is the Fourier Sine Series Not Defined at Discontinuous Points?

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SUMMARY

The discussion clarifies why the Fourier sine series is not defined at discontinuous points, specifically referencing Dirichlet's theorem. It highlights that the sine series for the function f(x) = cos(x) on the interval [0, π] converges to the average of the right and left-hand limits at discontinuities, while the cosine series for f(x) = sin(x) is defined at those points due to its continuity. The convergence behavior of these series is fundamentally linked to the nature of their extensions—odd for sine and even for cosine.

PREREQUISITES
  • Understanding of Fourier series, specifically sine and cosine series
  • Familiarity with Dirichlet's theorem
  • Knowledge of function continuity and discontinuity
  • Basic concepts of odd and even function extensions
NEXT STEPS
  • Study the properties of Dirichlet's theorem in detail
  • Learn about the convergence of Fourier series at discontinuities
  • Explore the differences between odd and even function extensions
  • Investigate the implications of continuity on Fourier series convergence
USEFUL FOR

Mathematicians, physics students, and engineers interested in signal processing, particularly those studying Fourier analysis and its applications in handling discontinuous functions.

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Why is Fourier sine series of any function satisfying Dirichlet's theorem, not defined on the discontinuous points whereas we define it for Fourier cosine series?

ex - sine series of f(x) = cosx, 0<=x<=∏ is defined on 0<x<∏

whereas cosine series of f(x) = sinx, 0<=x<=∏ is defined on 0<=x<=∏
 
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Abdul Quadeer said:
Why is Fourier sine series of any function satisfying Dirichlet's theorem, not defined on the discontinuous points whereas we define it for Fourier cosine series?

ex - sine series of f(x) = cosx, 0<=x<=∏ is defined on 0<x<∏

whereas cosine series of f(x) = sinx, 0<=x<=∏ is defined on 0<=x<=∏

Not sure what you are getting at. The half range Fourier series you mention both converge for all x. In the first case the FS converges to the average of the right and left hand limits at x = 0 of the odd extension of cos(x). In the second case the FS converges to sin(0) = 0 at x = 0. That is because the even extension of sin(x) is continuous at x=0 while the odd extension of cos(x) is not continuous at x = 0.
 

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