neutronpy.kmpfit.Fitter.parinfo

Fitter.parinfo

A list of dicts with parameter contraints, one dict per parameter, or None if not given.

Each dict can have zero or more items with the following keys and values:

'fixed': a boolean value, whether the parameter is to be held fixed or not. Default: not fixed.

'limits': a two-element tuple or list with upper end lower parameter limits or None, which indicates that the parameter is not bounded on this side. Default: no limits.

'step': the step size to be used in calculating the numerical derivatives. Default: step size is computed automatically.

'side': the sidedness of the finite difference when computing numerical derivatives. This item can take four values:

  • 0 - one-sided derivative computed automatically (default)
  • 1 - one-sided derivative (f(x+h) - f(x) )/h
  • -1 - one-sided derivative (f(x) - f(x-h))/h
  • 2 - two-sided derivative (f(x+h) - f(x-h))/2h
  • 3 - user-computed explicit derivatives

where h is the value of the parameter 'step' described above.

The “automatic” one-sided derivative method will chose a direction for the finite difference which does not violate any constraints. The other methods do not perform this check. The two-sided method is in principle more precise, but requires twice as many function evaluations. Default: 0.

'deriv_debug': boolean to specify console debug logging of user-computed derivatives. True: enable debugging. If debugging is enabled, then 'side' should be set to 0, 1, -1 or 2, depending on which numerical derivative you wish to compare to. Default: False.

As an example, consider a function with four parameters of which the first parameter should be fixed and for the third parameter explicit derivatives should be used. In this case, parinfo should have the value [{'fixed': True}, None, {'side': 3}, None] or [{'fixed': True}, {}, {'side': 3}, {}].