Curve fitting function doesn't work for (-tanh(w[0]))^x
Sandbo
Thu, 04/04/2019 - 12:34 pm
Hi,
On Igor 8.0.2.1, I am trying to write my own fitting function, which will look something like this: (given w[0] = 2)
f(x) = ((-tanh(w[0]))^x)
For some reasons, once I have the above in the expression, the curve fitting function becomes nan. The Graph Now button gives an empty plot.
I also tried:
w[0]^x works
((-tanh(w[0]))*x) works
w[0]*((-tanh(1))^x) also works
(-tanh(w[0]))^x fails
So it seems something is happening when I have power of a tanh. Appreciated if you can provide me with some ideas what the cause is.
The below is the function for the fitting if you would like to try:
Function secSqueeze(w,x) : FitFunc
Wave w
Variable x
//CurveFitDialog/ These comments were created by the Curve Fitting dialog. Altering them will
//CurveFitDialog/ make the function less convenient to work with in the Curve Fitting dialog.
//CurveFitDialog/ Equation:
//CurveFitDialog/ if (mod(x+2,2)>0)
//CurveFitDialog/ f(x) = 0
//CurveFitDialog/ else
//CurveFitDialog/ f(x) = (1/sqrt(cosh(squeezing)))*(-tanh(squeezing))^(x/2)*sqrt(factorial(2*(x/2)))/(2^(x/2)*factorial((x/2)))
//CurveFitDialog/ endif
//CurveFitDialog/ End of Equation
//CurveFitDialog/ Independent Variables 1
//CurveFitDialog/ x
//CurveFitDialog/ Coefficients 1
//CurveFitDialog/ w[0] = squeezing
////The actual function
// if (mod(x+2,2)>0)
// return 0
// else
// return ((1/sqrt(cosh(w[0])))*(-tanh(w[0]))^(x/2)*sqrt(factorial(2*(x/2)))/(2^(x/2)*factorial((x/2))))^2
// endif
// return ((1/sqrt(cosh(w[0])))*(-tanh(w[0]))^x)
return (-tanh(w[0]))^x
End
Wave w
Variable x
//CurveFitDialog/ These comments were created by the Curve Fitting dialog. Altering them will
//CurveFitDialog/ make the function less convenient to work with in the Curve Fitting dialog.
//CurveFitDialog/ Equation:
//CurveFitDialog/ if (mod(x+2,2)>0)
//CurveFitDialog/ f(x) = 0
//CurveFitDialog/ else
//CurveFitDialog/ f(x) = (1/sqrt(cosh(squeezing)))*(-tanh(squeezing))^(x/2)*sqrt(factorial(2*(x/2)))/(2^(x/2)*factorial((x/2)))
//CurveFitDialog/ endif
//CurveFitDialog/ End of Equation
//CurveFitDialog/ Independent Variables 1
//CurveFitDialog/ x
//CurveFitDialog/ Coefficients 1
//CurveFitDialog/ w[0] = squeezing
////The actual function
// if (mod(x+2,2)>0)
// return 0
// else
// return ((1/sqrt(cosh(w[0])))*(-tanh(w[0]))^(x/2)*sqrt(factorial(2*(x/2)))/(2^(x/2)*factorial((x/2))))^2
// endif
// return ((1/sqrt(cosh(w[0])))*(-tanh(w[0]))^x)
return (-tanh(w[0]))^x
End
Raising a negative number to a non-integral power gives a complex result:
(2.79367e-16,0.912481)
Your expression is not complex, so Igor returns NaN.
April 4, 2019 at 03:03 pm - Permalink