Levenberg Marquardt: how to use a decimal as an exponent? it only works with integers!

Question

Hi,
I use LV 6.1 and I want to use the Levenberg-Marquardt algorithm
to extract the best fit coefficients for the following equation:
Y =A*(X-B)^(1.7)
The data I input are X and Y (that I know)  and I
expect the vi to display A and B. I also input approximate values of A
and B. It works perfectly well for Y=A*(X-B)^2 or for any integer but
neither for (1,7), nor for  1.7 or (17/10).
Has anyone an idea how to use a decimal number as an exponent? Is there another vi I can us?
Any help appreciated,
Nicooo

Answer 1

Well, I don't think my initial parameters are the problem, because it works perfectly with a value of the exponent equal to 2 for example, but not for 2.1.
the vi computes 1000 iterations. Concerning the errors,for example  if I input 1.7 I will get  NaN for the coefficients and 0 for the error
If I input 1,7 as an exponent I get 0 for the coefficients but 23001 for the error.
I've attached two vis and somme typical data. I hope you will have a better idea than me
Thank you for your help
Attachments:
creep.vi ‏273 KB
creep2.vi ‏302 KB
04-12-08_X7142B-27_cl_3200s_pas2_50g_800C_2.txt ‏329 KB