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Correct Answer
FFT stands for Fast Fourier Transform, so the second 'F' stands for "Fourier". Joseph Fourier's (1768-1830) theorem states that any periodic signal with a frequency "f" can be expressed as a (weighted) sum of the sinusoids of frequency "f" and f's harmonics. Yes, even a square wave can be described accurately by a sum of sine waves - if you have enough of them. Square waves can be constructed by summing a sinusoid at the fundamental frequency, f, plus sinusoids at all of f's odd harmonics - 3f, 5f, etc.
