Dieter,
Thanks for finding this out and for finding the code behind it. My Lisp is
still pretty weak.
Rich
On Fri, Jan 29, 2010 at 4:02 PM, Dieter Kaiser <drdieterkaiser at web.de>wrote:
> Am Freitag, den 29.01.2010, 15:38 -0500 schrieb Richard Hennessy:
> > Is it really necessary to factor?
> >
> > (%i9) factor(x^10-1);
> > (out9) (x-1)*(x+1)*(x^4-x^3+x^2-x+1)*(x^4+x^3+x^2+x+1)
> > (%i10) signum(x^10-1);
> > (out10) signum(x^10-1)
> >
> > Can you turn this off? Signum() does not use the result is many cases
> > anyway.
> >
> > Rich
> >
> >
> > On Fri, Jan 29, 2010 at 12:42 AM, Richard Fateman
> > <fateman at cs.berkeley.edu> wrote:
> >
> >
> > What is maxima doing here that takes so long?
> > It is calling factor.
> >
> > do :lisp (trace factor)
> >
> >
> > Why? probably looking to take out a sign from one of the
> > factors?
> >
> > RJF
>
> It is not the function signum which does the factoring, but the function
> $sign. This function is called from the simplifying function simpsignum.
>
> (%i28) sign(x^10000+x);
> Evaluation took 5.8604 seconds (6.1778 elapsed) using 34.073 MB.
> (%o28) pnz
>
> This is the code which does it.
>
> (defun factor-if-small (x)
> (if (< (conssize x) 51.)
> (let ($ratprint)
> (declare (special $ratprint))
> (factor x)) x))
>
> I think the test to decide if we have a small expression is to weak.
> This might be a bug, but the problem is not related to the signum
> function. It is a more general problem.
>
> Dieter Kaiser
>
>
>