"genfit" function

[VladimirN]

I want to determine parameters for function "N*(Sigma^m)=10^C" by using "genfit" built-in function in Mathcad. However, in the process of calculating an error occurs. Whether it is possible to bypass this error somehow? And maybe the input data not absolutely approach under this function ("N*(Sigma^m)=10^C"), but I need to know parameters for it.

[RichardJ]

You have the indices reversed in your definition of f(x,D). Swap them, and it works fine.

[VladimirN]

For some reason "genfit" function is odd. I opened my worksheet with an error, and change the initial conditions to (1;1) as all recalculate and parameters were found (by the way, these initially value (1;1) were taken, but then all the same, an error occurred). But neither my nor your trick does not work in the worksheet in attachment.

[RichardJ · Accepted solution]

Your function is written in such a way that it's numerically unstable. For a good fit D2 needs to be a number greater than 300, and you have 10 raised to the power of D2. If the expression is changed to an equivalent expression with 10 raised to the power of D2/D1 it works.

[Werner_E]

Here are two different approaches - the results are slightly different: