Re: rational roots

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

On Tue, Sep 24, 2013 at 10:20:17AM -0700, somayeh didari wrote:
> I need to find rational roots of a polynomial in Pari/gp. But when I use polroots command, Pari gives all complex roots. On the other hand rational points are returned in complex type, too. How can i find rational roots in rational type? for example
> 
> ? polroots(x^2-0.25)
> %14 = [-0.5000000000000000000000000000 + 0.E-28*I, 0.500000000000000000000000000
> 0 + 0.E-28*I]~
> 
> But I need [-0.5, 0.5]!!!!!!!!!

If you want rational roots, you need to give rational coefficients.
Do
? nfroots(,x^2-1/4)
%1 = [-1/2,1/2]

which is a short-hand for
? Q=nfinit(Q);
? nfroots(Q,x^2-1/4)
%6 = [-1/2,1/2]

Cheers,
Bill.