Fourier series

**AkilMAI** · March 5th 2011, 08:50 AM

Define $f(t)=e^{-t} on [-\pi,\pi)$ and extend f to be $2\pi$ -periodic.Find the complex Fourier series of f.
Then, apply Parseval's relation to f to evaluate $/sum {1/(1+k^2)}$ .
I know how the complex fourier series looks like but I don't know how to extend it periodically

**TheEmptySet** · March 5th 2011, 09:01 AM

Originally Posted by AkilMAI

Define $f(t)=e^{-t} on [-\pi,\pi)$ and extend f to be $2\pi$ -periodic.Find the complex Fourier series of f.
Then, apply Parseval's relation to f to evaluate $/sum {1/(1+k^2)}$ .
I know how the complex fourier series looks like but I don't know how to extend it periodically

The function $e^{t}$ is not peroidic, but when you find its Fourier series on $[-\pi,\pi)$ and then let $t \in \mathbb{R}$ not just the interval you will be a new function that is periodic on all of $\mathbb{R}$ (It will just be translated copies of the series on the original interval)

**chisigma** · March 5th 2011, 09:12 AM

Originally Posted by AkilMAI

Define $f(t)=e^{-t} on [-\pi,\pi)$ and extend f to be $2\pi$ -periodic.Find the complex Fourier series of f.
Then, apply Parseval's relation to f to evaluate $/sum {1/(1+k^2)}$ .
I know how the complex fourier series looks like but I don't know how to extend it periodically

http://www.mathhelpforum.com/math-he...em-167764.html

Kind regards

$\chi$ $\sigma$

**mr fantastic** · March 5th 2011, 01:43 PM

Originally Posted by AkilMAI

Define $f(t)=e^{-t} on [-\pi,\pi)$ and extend f to be $2\pi$ -periodic.Find the complex Fourier series of f.
Then, apply Parseval's relation to f to evaluate $/sum {1/(1+k^2)}$ .
I know how the complex fourier series looks like but I don't know how to extend it periodically

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