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Sine series for cos(x) (FOURIER SERIES) 
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#1
Nov3009, 02:15 AM

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I was finally able to figure out how to find the sine series for cos(x), but only for [0,2pi]. A question i have though is what is the interval of validity? is it only [0,pi]?
Ie if I actually had to sketch the graph of the sum of the series, on all of R, would I have cosine or just a periodic extension of cosine from [0,2pi]? 


#2
Nov3009, 03:54 AM

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Hey Konrad, welcome to PF.
I am afraid I don't entirely understand your question. You say that you have managed to write cos(x) as a(n infinite) sum of sines on the interval [0, 2pi]. But both cos(x) and the sines you used are periodic with period 2pi, aren't they? So if the infinite sum converges to cos(x) on an interval with a length of at least one period, then it converges to cos(x) everywhere, doesn't it? 


#3
Nov3009, 01:15 PM

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Thanks
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P: 7,642

If you have expanded cos(x) in a sine series using [itex]p = 2\pi[/itex] in the formula
[tex] b_n = \frac 2 p \int_0^p \cos(x) \sin{\frac{n\pi x}{p}}\,dx[/tex] what you are representing is the [itex]4\pi[/itex] periodic odd extension of cos(x). [edit  corrected typo: b_{n} not a_{n}] 


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