Prove Lim N→∞ of Rudin Fourier Series 8.19

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SUMMARY

The discussion centers on proving the limit of the Fourier series as described in Rudin's problem 8.19. Specifically, it establishes that for a continuous function \( f \) on \( \mathbb{R} \) satisfying \( f(x + 2\pi) = f(x) \) and where \( a/\pi \) is irrational, the limit as \( N \) approaches infinity of the sum \( \sum_{n=1}^{N} f(x + na) \) equals \( \frac{1}{2\pi} \int_{-\pi}^{\pi} f(t) dt \) for every \( x \). The discussion emphasizes starting the proof with the function \( f(x) = e^{ikx} \) as a foundational step.

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  • Understanding of Fourier series and their convergence properties
  • Familiarity with continuous functions and periodicity
  • Knowledge of complex exponentials, specifically \( e^{ikx} \)
  • Basic calculus, particularly integration over intervals
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  • Explore the implications of irrational multiples in periodic functions
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Rudin 8.19
f is a continuous function on R, f(x+2Pi)=f(x), and a/pi is irrational.
Prove that

lim N goes to infinity (Sum n=1,...,N f(x+na)) =(1/2pi) * [tex]\int[/tex] f(t)dt from -pi to pi
for every x.

Hint: do it first for f(x)=exp(ikx)

THANKS!
 
Last edited:
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welcome to pf!

hi 220205! welcome to pf! :wink:

hint: do it first for f(x)=exp(ikx) :smile:
 


tiny-tim said:
hi 220205! welcome to pf! :wink:

hint: do it first for f(x)=exp(ikx) :smile:

Thanks!
 

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