I am trying to fit some data to a model.

The model is given by this equation.

A*(C/B)^(2*B)*(B-1)^(2*(B-1))/(205*10^9*(2*B+1)*(sigma^2-(650*10^6)^2)*sigma^(2*(B-1)))+(C-sigma)^2*A/(205*10^9*(2*B+1)*(sigma^2-(650*10^6)^2))

> with(Statistics);
> X := Vector([819.4, 795.6, 788.8, 782.0, 776.56, 763.64, 748.28, 724.42, 717.40, 711.28, 707.20, 680], datatype = float);
> Y := Vector([5.3*10^4, 7.8*10^4, 9*10^4, 9.5*10^4, 10^5, 1.2*10^5, 1.37*10^5,...