acosh, getting consistent results from GSL and Guile

[Marco Maggi]

Ciao,

  on the GNU Scientific Library mailing list I asked the
following:

>for  some values of  the operand  I get  different
>results from  'acosh' when  using  GSL (1.8), Guile (1.8)
>and GNU Octave (2.1.63). Other functions give me
>different results, too. [...]
>
>Browsing Wikipedia for acosh  formulas it seems to me
>that GSL is using an equivalent of:
>
>    (log (- x (sqrt (- (* x x) 1))))
>
>while the others are using an equivalent of:
>
>    (log (+ x (sqrt (- (* x x) 1))))
>
>I'm interested in having equal results from GSL
>and Guile, [...]

and got the following reply:

>There are different branch cut conventions used
>by different programs. acosh or other inverse
>functions are mutiple-valued functions so the
>result is not uniquely defined.
>
>GSL uses one of the two common conventions
>(W.Kahan's) because it has better consistency with
>IEEE arithmetic.  It is also used by Common
>Lisp -- see the GSL Reference manual for details and
>references.  The others are using a different
>convention (Abramowitz & Stegun).  As such, all the
>answers are correct.

So, do you suggest me to override the GSL function to
make it equal to the Guile one, or vice versa?


--
Marco Maggi

"They say jump!, you say how high?"
Rage Against the Machine - "Bullet in the Head"