[AstroPy] non-linear least square fitting in python

[gonghang.naoc]

Hi all,
We can use scipy.optimize.leastsq to do least square fitting in python.
However we need to make an initial guess. (
http://python4mpia.github.io/fitting_data/least-squares-fitting.html). So I
am not sure we can get a global optimum.

There is other simple way we can fit a *y=a*e^-(b*x)+c* curve? My data are
Xn, Yn and ERRORn. A constant c makes the fitting difficult since we can
not make a log on both sides and use linear fitting any more.

Thank you.
best
Hang
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