The Organization of Minuit.

The Minuit package acts on a multiparameter Fortran function to which we give the generic name FCN, although the actual name may be chosen by the user. This function must be defined and supplied by the user (or by an intermediate program such as HBOOK[2] or PAW[3], in case Minuit is being used under the control of such an intermediate program). The value of FCN will in general depend on one or more variable parameters whose meaning is defined by the user (or by the intermediate program), but whose trial values are determined by Minuit according to what the user requests should be done to FCN (usually minimize it). To take a simple example, suppose the problem is to fit a polynomial through a set of data points. Then the user would write an FCN which calculates the chisquare between a polynomial and the data; the variable parameters of FCN would be the coefficients of the polynomials. Using Minuit commands, the user would request Minuit to minimize FCN with respect to the parameters, that is, find those values of the coefficients which give the lowest value of chisquare. The user must therefore supply, in addition to the function to be analyzed, a set of commands to instruct Minuit what analysis is wanted. The commands may be given in several different forms:

• As a data file, corresponding to the traditional ``data cards'', for batch processing;
• Typed in at execution time at a terminal, for interactive running;
• Coded in Fortran in the calling program, which allows looping, conditional execution, and all the other possibilities of Fortran, but not interactivity, since it must be compiled before execution. This is sometimes known as running Minuit in ``slave mode''. HBOOK and PAW use Minuit in this way.

It is also possible to mix any of the above forms, for example starting off a fit with a standard command file, then turning it over to the interactive user for the final command steps.