Nyquist theorem: To reconstruct a signal without aliasing, fs > 2·fin.

Oversampling: fs > 2·fin — output = fin.
Nyquist: fs = 2·fin — boundary condition.
Undersampling: fs < 2·fin — signal aliases to falias = |fin mod fs| or fs − (fin mod fs). Intentional undersampling (bandpass sampling) is used in IF digitisers and software-defined radio.
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