# Periodicity using Fourier transform

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aurc89 on 18 Nov 2016
Commented: Walter Roberson on 18 Nov 2016
Hi everyone, I have a simple question concerning how to find the periodicity of an oscillating function using the Fourier transform. Let's suppose to have an oscillating function y=f(x) where x and y are two vectors and x is the time vector expressed in second (s). Can you suggest me a code to retrieve the frequency (or frequencies) of this function, expressed in herz (i.e. 1/s) by using the Fourier transform? Thanks a lot.
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aurc89 on 18 Nov 2016
In this case uniform but sometimes could be irregular in fact

Walter Roberson on 18 Nov 2016
If the times are uniform intervals then do peak finding on fft(y-mean(y))
See the fft example for how to calculate the frequency from bin and fs and length
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Walter Roberson on 18 Nov 2016
For x at equal time intervals,
L = length(y);
Fs = 1/(x(2)-x(1));
f = Fs*(0:(L/2))/L;
Now the first length(f) positions in fft(y-mean(y)) are associated with corresponding frequencies in f, so if you find a peak at position K then the frequency corresponding is f(K)