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#### 15.5.455 power

`power(x [, axis, /power, /shape, /onedim])`

Returns the power spectrum of `x` along the indicated `axis` (which defaults to 0), with power at negative frequencies transferred to the corresponding positive ones. By default, each element of the power spectrum is equal to the square of the amplitude of the variation at the corresponding frequency, and element zero is equal to the square of the average value of `x`. If you wish to add several elements of such a power spectrum, then you have to multiply the first and last elements by a factor of 2 first. This use is flagged by keyword `/power`. e.g., for this implementation of `power`, Parseval’s theorem reads: ```mean(x^2) = total(p(1:*-2))/2 + p(0) + p(*-1),``` with `p = power(x)`, or `mean(x^2) = total(power(x,/power)).` If you wish to investigate the shape of the power spectrum, then you need to do the `/power` correction twice to also correct for the absence of a negative frequency contribution to the first and last elements (whereas all other elements do have power from a corresponding negative frequency added in). This use is flagged by the `/shape` keyword. If you specify `/onedim` but no `axis`, then `x` is regarded as a one-dimensional array.