let us asuume this discrete signal:(adsbygoogle = window.adsbygoogle || []).push({});

f(n)=a^n * u(n) ; where u(n) is unit step function

; u(n)=1 where n>=0

u(n)=0 where n<0

;0=<a<1

and the foruier transform for discrete signals is defined as :

F(i)=sum ( f(n)*e^(-j2*pi*n/N) ;n=0 to inifinity

i know that the sum is equal to:

F(i)=sum(1/(1-a*e^-j2*pi*n/N)

but actually i don't know why! could anyone help!

thanks in advance!

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# Fourier transform for Discrete signal

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