The various standard fit functions are defined by the commands

FPOL (polynomial or square root of a polynomial,
      optionally multiplied by an exponential function of a polynomial),
FLEG (sum of legendre polynomials, optionally consisting of even or odd terms only and normalized),
FEXP (sum of linear exponentials),
FPEAKS (Gaussian or Lorentzian peaks),
FMY defines a user defined fit function.


Data background

 
Any desired function can be considered as a background function since
the command FLAST allows the total fit function to be composed of
an unlimited number of standard or user defined components.
With FLAST/KEEP the last defined fit function is stacked
(no matter if the fit was executed or not), i.e.
the function specified  next is taken as an  additional term; successive
fits adding  components step  by  step  may  be executed  on  different
windows. Each stacked  function gets an internal serial  number by which
it is identified for deleting  or displaying singly.
Automatic renumbering is done after a component i is deleted by FLAST/DEL(i)
Keeping the last fit as background, the parameters of the respective
function(s) are not fixed automatically.


Peaks
 
 
Gaussian or Lorentzian peaks are specified by the command FPEAKS in three ways:
 
1. limits are found by the automatic peak search routine,
2. limits or centres are given as prompting input by cursor or terminal,
3. peak regions are defined by the global fit windows (command FWIN).
 
Whenever peaks are defined by limits,
initial values for the peak parameters area, centre and full width at half maximum
are derived from sum, mean and variance of the net data in the window referred to.
If only centres are given as prompting input, the full width at half maximum must be specified;
the product of width and peak height yields the initial area.
The resulting function is displayed (unless the keyword NODISP is given).
Lower limits of areas and widths are set to zero;
limits for the peak centres are set equal to the window limits
or at least half the corresponding width above or below the centre, respectively.