The user must always supply a Fortran subroutine which calculates the function value to be minimized or analyzed.
CALL FCN (NPAR,GRAD,FVAL,XVAL,IFLAG,FUTIL)
EXTERNALand provided by the user).
Note that when Minuit is being used through an intermediate package such as HBOOK or PAW, then the FCN may be supplied by the this package.
The name of the subroutine may be chosen freely (in documentation we
give it the generic name FCN) and must be declared
EXTERNAL in the
user's program which calls Minuit (in data-driven mode) or calls
Minuit subroutines (in Fortran-callable mode).
The meaning of the parameters
XVAL is of course defined by
the user, who uses the values of those parameters to calculate his
The starting values must be specified by the user
(either by supplying parameter
definitions from a file, or typing them at the terminal,
in data-driven mode; or
by calling subroutine MNPARM in Fortran-callable mode),
and later values are determined by Minuit as it searches for the
minimum or performs whatever analysis is requested by the user.
FUTIL represents the name of a function or subroutine which may
be defined and supplied by the user and called from FCN.
If the user does not use the FUTIL feature, the last argument
may be given as zero, but if used, the name of FUTIL must
EXTERNAL and a subprogram of that name must be
supplied at loading time.
It is possible, by giving them different names, to analyze several different FCNs in one job. However, one analysis must be completed before the next is started. In order to avoid interference between the analyses of two different FCNs, the user should call Minuit (in data-driven mode) or MNINIT (in Fortran-callable mode) each time a new FCN is to be studied.