Wiling to do a curve fitting within Stan, I came along Eigen’s Levenberg-Marquardt Algorithm.

I nice way to use the routines are shown in:

I would code my optimization function directly in the routine as shown in the stackoverflow example.

Now my question is:

Are the derivates required? In other words what does the function signature in Stan may look like?

Is the `var`

implementation required or is just implementing the `double`

version enough?

(I suppose the derivatives are not needed, since min f^2 force them be to 0)