Re: poolroots division by zero

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

On Sat, Aug 11, 2012 at 02:07:11AM +0200, Sören Lennart Berg wrote:
> I'm getting a 'division by zero' from polroots for the following input:
> 
> ? polroots(64.0*z^6-640.0*z^5+2640.0*z^4-5760.0*z^3+7040.0*z^2-4608.0*z+1280.0)
>   ***   at top-level: polroots(64.0*z^6-64
>   ***                 ^--------------------
>   *** polroots: division by zero
>   ***   Break loop: type 'break' to go back to GP
> 
> using integer coefficients everythings just fine:
> ? polroots(64*z^6-640*z^5+2640*z^4-5760*z^3+7040*z^2-4608*z+1280)
> %1 = [2.0000000000000000000000000000000000000 + 0.E-38*I, 2.0000000000000000000000000000000000000 + 0.E-38*I, 2.0000000000000000000000000000000000000 + 0.E-38*I, 2.0000000000000000000000000000000000000 + 0.E-38*I, 1.0000000000000000000000000000000000000 - 0.50000000000000000000000000000000000000*I, 1.0000000000000000000000000000000000000 + 0.50000000000000000000000000000000000000*I]~
> 
> For your interest: the library function roots0() with flag set to 1 works/doesn't crash.

Hello Sören,

I believe this is bug #1309 which is fixed in pari-2.5.2 and the development branch.

   10- sqrt(0.E-97-1.12-97*I) -> div by 0 [#1309]                          [F76]

see <http://pari.math.u-bordeaux1.fr/Bugs/1309>
so I suggest you to upgrade to pari-2.5.2.

(The bug first appeared in pari-2.4.2.alpha)

Cheers,
Bill.