Nyquist says in order to recorver any signal you must sample it 2 times de frequency of the signal. So when you aply the Fourier transform to a periodic signal, will be non periodic and discrete in frequencies,and if you obey the Nyquist law you will not have aliasing, so you can apply a filter like a Chebyshev filter and make the inverse Fourier and obtain only the desired frequency, thats the base of tecomunications,greetings from Portugal
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