[Octave-bug-tracker] [bug #38063] optim: expfit inaccurate (incorrect)

[Muhali]

Follow-up Comment #2, bug #38063 (project octave):

>> - The example in the report fits a degree-1 model to data produced with a
degree-2 mode

This doesn't matter. You get similar (bad) expfit results with degree-1 data,
e.g. y=-0.5*exp(2*x). Moreover, with deg=2 expfit produces non-real
parameters. 

>> - Although general nonlinear fitting gave a good fit, the determined model
parameters (for a degree-2 model) were vastly incorrect

That is simply because the default MaxIter parameter in nonlin_curvefit is set
to 20; increasing that to 1000 gives a perfect fit (in reasonable time).

Again, if there is no noise there is not much to fit, and real-world cases
look more like the figure attached to the original post, for which expfit gets
it totally wrong.

I suggest to at least add a warning that expfit should not be used for
classical fitting problems. How about renaming it to e.g. prony_fit.m?

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