## Providing the Function to be Minimized

You must provide n functions of p variables for the minimization algorithms to operate on. In order to allow for general parameters the functions are defined by the following data types:

Data Type: gsl_multifit_function
  This data type defines a general system of functions with parameters.
  `int (* f) (const gsl_vector * x, void * params, gsl_vector * f)`
    this function should store the vector result f(x,params) in f for argument x and parameters params, returning an appropriate error code if the function cannot be computed.
  `size_t n`
    the number of functions, i.e. the number of components of the vector f
  `size_t p`
    the number of independent variables, i.e. the number of components of the vectors x
  `void * params`
    a pointer to the parameters of the function

Data Type: gsl_multifit_function_fdf
  This data type defines a general system of functions with parameters and the corresponding Jacobian matrix of derivatives,
  `int (* f) (const gsl_vector * x, void * params, gsl_vector * f)`
    this function should store the vector result f(x,params) in f for argument x and parameters params, returning an appropriate error code if the function cannot be computed.
  `int (* df) (const gsl_vector * x, void * params, gsl_matrix * J)`
    this function should store the n-by-p matrix result J_ij = d f_i(x,params) / d x_j in J for argument x and parameters params, returning an appropriate error code if the function cannot be computed.
  `int (* fdf) (const gsl_vector * x, void * params, gsl_vector * f, gsl_matrix * J)`
    This function should set the values of the f and J as above, for arguments x and parameters params. This function provides an optimization of the separate functions for f(x) and J(x) -- it is always faster to compute the function and its derivative at the same time.
  `size_t n`
    the number of functions, i.e. the number of components of the vector f
  `size_t p`
    the number of independent variables, i.e. the number of components of the vectors x
  `void * params`
    a pointer to the parameters of the function