Help in understanding the Power Spectral Density estimates using FFT

I came across this article in documentation which has this particular line:

psdx(2:end-1) = 2*psdx(2:end-1);

I couldn't quite get why only the indices 2 to last but one is taken i.e, first and last indices are discarded.

It would be great if anyone could help me understand why this is being done.

Answer 1

[Mathieu NOE]

hi

the explanation is just above

Zero frequency (DC) and the Nyquist frequency do not occur twice.

thats why the factor 2 on the psdx amplitude does not apply to indice "1" and "end"

[skrowten_hermit]

Ah. Thanks a lot. Understood now.