# What are the frequencies when N in fft(x,N) is odd?

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Ray Lee 10 Jul 2014
편집: Honglei Chen 10 Jul 2014
I know when N is even, fft(x,N) corresponds to frequencies 0,df,...,N/2*df,-(N/2-2)*df,...,-df.
What about if N is odd?

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### 채택된 답변

Honglei Chen 10 Jul 2014
편집: Honglei Chen 10 Jul 2014
In general, you can do it as
f = (0:N-1)*fs/N
If you want to map it to negative frequencies
if N is even
f(N/2+1:end) = f(N/2+1:end)-fs % sample -fs/2
or
f(N/2+2:end) = f(N/2+2:end)-fs % sample fs/2
if N is odd
f((N+1)/2+1:end) = f((N+1)/2+1:end)-fs
HTH
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Honglei Chen 10 Jul 2014
That's not true. Only one point is changed, from fs/2 to -fs/2 and because the result is periodic, they are indeed the same so you can mark it either way

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### 추가 답변(1개)

Michael scheinfeild 10 Jul 2014
if x is ampled at fs you can generate frequency vector use
freqvec=linspace(0,fs,N);
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Honglei Chen 10 Jul 2014
This is incorrect. linspace always sample the last one, which you don't want.

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