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

[Alexey A. Illarionov]

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Hi,

Just curious, how is this form preferable than the other?

In my opinions none of the forms avoid the loss of precision when f
and f_new are close.

Brian Gladman wrote:
> Update of bug #39713 (project gsl):
> 
> Status:          Ready For Test => Fixed
> 
> _______________________________________________________
> 
> 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).
> 
> 
> _______________________________________________________
> 
> Reply to this item at:
> 
> <http://savannah.gnu.org/bugs/?39713>
> 
> _______________________________________________ Message sent via/by
> Savannah http://savannah.gnu.org/
> 
> 


- -- 
С уважением, Алексей Александрович Илларионов.
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