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Re:Generalizations of Fourier Analysis by jdhwosnhw on Saturday April 15, 2023 @09:22PM (#35586383)
From a physics perspective, complex frequency results in “evanescent waves” - ie, waves that decay rather than oscillate (technically a fully complex frequency of the form a+ib will both oscillate and decay)
Re:Generalizations of Fourier Analysis by nextaccountic on Sunday April 16, 2023 @03:48AM (#35588080)
Yep: e^iω (with ω real) is an oscillation, but e^σ (with σ real) is an exponential decay when σ < 1, an exponential growth with σ > 0 and constant with σ = 1

so e^(σ + iω) = e^σ * e^iω is just an exponential growth or decay modulated by a sinusoid.. or, if σ is one, is just a pure oscillation

ω is the usual frequency, but σ + iω is the complex frequency. the fourier transform deals with function that receives ω as input, and the laplace transform deals with functions that receives σ + iω instead.

so the fourier transform is just a special case of the laplace transform with σ = 0

Re:Generalizations of Fourier Analysis by xeonmc on Sunday April 16, 2023 @04:28AM (#35588221)
> so the fourier transform is just a special case of the laplace transform with σ = 0

Another useful way of looking at it: Laplace transform is doing many extra Fourier-transform-but-with-decay giving you a map of which "global decay timescale" fits your data best -- since each "slice" is itself sufficient to fully describe the time series

They are all cases of integral transforms with different choices of the set of "primitive fingerprints" -- see chirp transform, wavelet transform, chirplet transform etc -- all taking advantage of the fact that if you choose one set of basis "brushes" that are not redundant with each other (e.g. having red-green-blue brushes is independent, as is magenta-green-yellow but having red-green-blue-yellow is not) then you will be able to describe your signal in terms of a composition of those kernels.

Re:Generalizations of Fourier Analysis by nextaccountic on Sunday April 16, 2023 @11:36PM (#35596602)
So, so I'm going with a rusty knowledge of a computer engineering course from years ago,

> a map of which "global decay timescale" fits your data best

What do you mean by this?

> -- since each "slice" is itself sufficient to fully describe the time series

But each slice is multiplied by an exponent.. which, okay, becomes a convolution that lets you recover the original function

Re:Generalizations of Fourier Analysis by xeonmc on Monday April 17, 2023 @03:20AM (#35597684)
Like how the DC term of Fourier transform is the baseline average, i.e. global energy constant (per time), the baseline decay describes the global energy dissipation or amplification (per time).