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## Main Question or Discussion Point

I want to find the maximum of the total of a list of numbers, of which each depends on some variable x. I'm taking the discrete Fourier transform of a set of points of a function, and trying to maximise the sum of the resulting list. Although I can plot the sum of the list as a function of x, when I try to maximise it Mathematica tells me

*Fourier::fftl:Argument{my list} is not a non-empty list or rectangular array of numeric quantities*. The error seems to be associated with taking the discrete fourier transform of a non-numerical quantity. It should be taking this Fourier transform with a range of possible numbers and finding the maximum, but seems to be getting it wrong. Can anyone help?