Re: real polynomials

[John Cremona <john.cremona@gmail.com>]

Thanks for the helpful comments.  I don't need multiplicities, and in
fact all I need is to know whether there are any roots at all in
(-oo,oo) or (-oo,0] or [0,oo).
At present the coeffs are converted from C doubles.  My pari
programming skills are rather basic so almost all the program uses
plain C types and I only convert to GEN for this one test.  Perhaps I
should be more brave...

John

On 6 November 2015 at 12:36, Karim Belabas
<Karim.Belabas@math.u-bordeaux.fr> wrote:
> * John Cremona [2015-11-06 12:13]:
>> I am using the library functions sturm() and sturmpart() to get the
>> number of real roots of a real polynomial in the whole real line or
>> just the positive or negative half-lines.  But occasionally it fails
>> with
>>
>>  ***   domain error in polsturm: issquarefree(pol) = 0
>>
>> which I expect is only happening if the discriminant is exactly 0.
>
> No, it can occur due to round-off errors whenever the input is inexact
> and two roots are "close enough".
>
> The documentation (??polsturm) specifies that the input must be
> squarefree. (And, in 2.8.*, that it better be exact...)
>
>> Is there a reliable way of taking the squarefreepart, or
>> the radical, or a real polynomial?
>
> Not if the input is inexact.
>
>> It is in fact true that in my program the polynomial coefficients are
>> always rational with denominator a power of 2, so will be known
>> exactly.
>
> In that case, the above problems don't occur. Do you need the number of
> real roots including multiplicity ? If not, P /= gcd(P,P') is enough.
>
>> I would like to avoid having to keep track of the
>> denominators and using (scaled) integer coefficients.
>
> Just use rational coefficients, sturm/sturmpart will cope.
>
> If you're using 2.8.* and library mode, you should try
> ZX_sturmpart / ZX_sturm  (sturmpart itself is now deprecated).
>
> Cheers,
>
>     K.B.
> --
> Karim Belabas, IMB (UMR 5251)  Tel: (+33) (0)5 40 00 26 17
> Universite de Bordeaux         Fax: (+33) (0)5 40 00 69 50
> 351, cours de la Liberation    http://www.math.u-bordeaux.fr/~kbelabas/
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