[Bug-gsl] [bug #39713] roots/secant.c "derivative value is not finite" for a good guess

[Brian Gladman]

Update of bug #39713 (project gsl):

                  Status:          Ready For Test => Fixed                  

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Follow-up Comment #17:

Thanks for testing the fix on Linux, which allows me to mark it as fixed.

The form df_new = df * (1 - f_new / f) is better than the earlier expression
because it avoids the severe loss of precision when (x_new - x) is used in the
demominator of the expression for df_new (this was the primary cause of the
problems you experienced).

In fact I have just made a further change to:

   df_new = df * ((f - f_new) / f);

which avoids the possible loss of precision in using (1 - f_new / f).


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