gsl_root_test_residual(double f, double epsabs)

[Karsten Howes]

Hi,

I am working with the 1D root finder framework and I am trying to figure 
out how to get the value of my function at the current best root estimate 
(without re-evaluating my function of course).  I need it because I want 
to stop when the function is sufficiently close to zero and I dont 
actually care about the exact location of the root.  The information does 
not appear to be in the solver state:

typedef struct
   {
     const gsl_root_fsolver_type * type;
     gsl_function * function ;
     double root ;
     double x_lower;
     double x_upper;
     void *state;
   }
gsl_root_fsolver;

How do I use the function gsl_root_test_residual?

If it doesn't work, one possibility is for me to maintain a map x->f for 
all previous valuations of f and then just look up f in the map using the 
current best root.  Sorry if I'm missing something obvious.

Thanks for a great library Brian et al.

Best regards,
Karsten