forum.alglib.net

I tried modifying the "lsfit_d_nlf" example of the .net ALGLIB library under C# and VB to fit a single-exponential decay curve.

y=a+b*exp(-c*x)

Please see the attached figure for the details.
Basically, I first generate the data with pre-specified coefficients a=1.4, b=4.5, c=0.54 under the two platforms, and do the unweighted fitting.
Under C#, the library works flawlessly and gives me (a,b,c)=(1.4,4.5,0.54).
Under VB, the fitted results are (a,b,c) = (1.25,3.72,0.39), which is far from desirable.

Does anyone know why there is such a discrepancy? Any suggestion to solve this issue will be appreciated!