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24-Ruby III
VladimirN24-Ruby III
24-Ruby III
October 30, 2015
Solved

"genfit" function

  • October 30, 2015
  • 1 reply
  • 2512 views

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.

Best answer by RichardJ

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.

1 reply

R
RichardJ19-Tanzanite
19-Tanzanite
October 30, 2015

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

V
24-Ruby III
VladimirN24-Ruby IIIAuthor
24-Ruby III
October 30, 2015

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.

Pic_1.png

R
RichardJ19-TanzaniteAnswer
19-Tanzanite
October 30, 2015

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.

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