Re: gnuplot numerical errors on Intel MacBook

[CM <mon...@gm...>]

On 9/24/06, Hans-Bernhard Br=F6ker <br...@ph...> wrote:
>
> CM wrote:
>
> > No, that's not what I'm seeing.  I'm seeing a graph with dotted lines
> > but no function output.  I have posted one more picture, this time the
> > result of running
> >
> > plot [1:2] gamma(x)
> >
> > It's rather interesting, and I'm afraid not terribly enlightening.
>
> Actually, it is.  It matches my original guess perfectly.  It shows
> exactly what you would get if signgam is random garbage, which just so
> happens to equal -5.58e8.  This is because the implementation of the
> user-visible function gamma() is internally:
>
>         double gamma(gamma x) {
>              return exp(GAMMA(x)) * signgam;
>          }
>
> [see specfun.c:f_gamma()], where GAMMA is either lgamma(), gamma(), or
> gnuplot's own lngamma(), depending on availability.  The correct values
> of gamma(1) and gamma(2) are 1.0, yours are -5.58e08, but other than
> that, the shape is quite right.  I get exactly your plot, but for
>
>       plot [1:2] -5.58e8*gamma(x)
>
> Please check your config.log's lines about signgam.



configure:8573: checking whether signgam is declared
configure:8607: gcc -c -g -O2   -I/usr/X11R6/include conftest.c >&5
configure:8613: $? =3D 0
configure:8620: test -z "$ac_c_werror_flag" || test ! -s conftest.err
configure:8623: $? =3D 0
configure:8630: test -s conftest.o
configure:8633: $? =3D 0
configure:8645: result: yes

Is this what you expected?

The true test would be that while gamma() is wrong, lgamma() plots
> correctly.


Aha.  Yes.  lgamma does, in fact, do the right thing.

- C