I don't see how this is relevant. Certainly related, but not immediately relevant. The Nyquist rate / frequency (depending on where you stand) determines whether the process of sampling is invertible, but the FFT exists independent of the process of sampling and is always invertible (in the maths). 'acchow's question is whether this holds in practical systems, like computing the FFT and IFFT with the finite precision + finite range numbers that we can use.