Andreas Enge on Tue, 24 Sep 2013 19:42:00 +0200 |
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Re: rational roots |
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]!!!!!!!!! Try factor, and prefer to specify the coefficient as a rational if you look for rational roots: ? factor (x^2-1/4) %1 = [2*x - 1 1] [2*x + 1 1] ? factor (x^2-0.25) %2 = [x + 0.50000000000000000000000000000000000000 1] [x - 0.50000000000000000000000000000000000000 1] seems to work and look for real roots. Otherwise, you still have the option to round rounding errors away: ? real (polroots (x^2-0.25)) %3 = [-0.50000000000000000000000000000000000000, 0.50000000000000000000000000000000000000] Andreas