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Complex Variables and Applications
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Author: James Ward Brown And Ruel V Churchill
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You're a long way from Fourier transforms. Start with basic complex numbers. The classic text that I know of is RV Churchill: https://www.amazon.com/Complex-Variables-Applications-James-Churchill/dp/9339205154/ref=sr_1_7?ie=UTF8&qid=1516220452&sr=8-7&keywords=churchill+complex+variables. The code you posted is correct as far as it goes, but it needs to do a lot more for FFT.
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