The fft results in a spectra that includes positive and negative frequencies. Don't ask me to explain what a negative frequency really means but the negative frequency esults are just the conjugates of the positive frequency results. They are not needed for the PSD calculation which is why they are rmeoved by the following line:
You have to account for the enrgy lost when this portion of the spectra is removed so we multiply the rest by 2. The 0Hz and Nyquist frequency results don't have an imaginary portion and are not included in the negative frequency portion so they are not muiltiplied by 2.
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