In a book the Fourier transform is defined like this. Let g(t) be a nonperiodic deterministic signal... and then the integrals are presented. So, I understand that the signal must be deterministic and not random. But why it has to be nonperiodic (aperiodic). The sin function is periodic and we can calculate its Fourier transform. Is it because a nonperiodic signal is absolutely integrable? And with the sin function. Yes, I can calculate. But deltas appear. This is the answer?