[Help-gsl] Multidimensional Minimisation

[Alexandros Beskos]

I try to use the multidimensional minimisation algorithms
and I am conserned about the parameters they use.
So the algorithms that use the gradient for the target function, say f,
use (among others) the command:

int gsl_multimin_fdfminimizer_set (gsl_multimin_fdfminimizer * s, 
gsl_multimin_function_fdf *fdf, const gsl_vector * x, double step_size, 
double tol)

What these algorithms do is to pick up a certain one-dimensional 
direction after considering the value, or the gradient of f at the 
current estimate of the minimum and then apply one of the available 
one-dimensional techniques. Now, the command I have written above is 
supposed to provide the parameters "tol" and "step size" for the 
directional one-dimensional minimisation. What I cannot understand is 
the meaning of the parameter "step_size". The one-dimensional methods 
are supposed to somehow bracket the minimum in an interval and in theory
require as parameters the boundaries of such an interval. In a more 
advanced level there is also an algorithm that generates such an 
interval given two points. Is this what the  "step_size" is doing ?
Provide the second point for the initiation of the above interval 
identification algorithm ??