[GAP Forum] RootsOfPolynomialAsRadicals - nffactor: the PARI stack overflows

[Andreas Distler]

Dear Daniel,

I cannot reproduce your problem. Which version of PARI/GP are you using?

Anyway, you can use the function SetPariStackSize from the Alnuth 
package to increase the amount of memory PARI/GP will use. The function 
takes a positive integer as argument specifying the amount of memory in 
MB. The default value is 128 MB as you may have guessed.

It's an oversight that SetPariStackSize is not documented. I will change 
this in the next version of Alnuth. Thanks for pointing me to this 
shortcoming.

Best wishes,

    Andreas

On 07/05/13 21:31, Daniel Blazewicz wrote:
> Hi,
>
> I use RootsOfPolynomialAsRadicals from RadiRoot package to compute roots of some simple polynomials of degree 5 and 6. In many cases mentioned method works excellent and produces very nice formulas. However for few specific polynomials it fails with stack overflow error. This is fully deterministic behavior on my environment, e.g. for f(x) = x^6+30*x+93 :
>
>
>   ┌───────┐   GAP, Version 4.6.2 of 02-Feb-2013 (free software, GPL)
>   │  GAP  │   http://www.gap-system.org
>   └───────┘   Architecture: i686-pc-linux-gnu-gcc-default32
>   Libs used:  gmp, readline
>   Loading the library and packages ...
>   Components: trans 1.0, prim 2.1, small* 1.0, id* 1.0
>   Packages:   AClib 1.2, Alnuth 3.0.0, AtlasRep 1.5.0, AutPGrp 1.5, Browse 1.8.2, CRISP 1.3.5, Cryst 4.1.11, CrystCat 1.1.6, CTblLib 1.2.1,
>               FactInt 1.5.3, FGA 1.2.0, GAPDoc 1.5.1, IO 4.2, IRREDSOL 1.2.1, LAGUNA 3.6.3, Polenta 1.3.1, Polycyclic 2.10.1, RadiRoot 2.6,
>               ResClasses 3.3.0, Sophus 1.23, TomLib 1.2.2
>   Try '?help' for help. See also  '?copyright' and  '?authors'
> gap> x := Indeterminate( Rationals, "x" );;
> gap> f := UnivariatePolynomial( Rationals, [93,30,0,0,0,0,1]);;
> gap> RootsOfPolynomialAsRadicals(f, "latex", "x6_30x_93");;
>    ***   at top-level: ...n(),[2,4,3])>=0,fac=lift(nffactor(f,pol)),if(
>    ***                                             ^--------------------
>    *** nffactor: the PARI stack overflows !
>    current stack size: 128000000 (122.070 Mbytes)
>    [hint] you can increase GP stack with allocatemem()
>
>    ***   at top-level: for(i=1,#fac[,1],for(j=1,fac[i,2
>    ***                             ^--------------------
>    ***   _[,_]: not a matrix.
> Error, List Element: <list>[1] must have an assigned value in
>    faktoren[1] := lcoeff * faktoren[1]; called from
> FactorsPolynomialPari( AlgExtEmbeddedPol( H, poly ) ) called from
> FactorsPolynomialAlgExt( erw.H, CyclotomicPolynomial( Rationals, i ) ) called from
> RR_RootOfUnity( erw, DegreeOverPrimeField( erw.K ) ) called from
> CallFuncList( RootsOfPolynomialAsRadicalsNC, arg ) called from
> <function "RootsOfPolynomialAsRadicals">( <arguments> )
>   called from read-eval loop at line 3 of *stdin*
> you can 'return;' after assigning a value
> brk>
>
>
> Other examples:
> f(x) = x^6+105*x+237
> f(x) = x^6+243*x+513
>
> I don't know how to work around this problem. Tried to increase workspace size but it did not solve the issue. I will be grateful for your help.
>
> With regards,
> Daniel
>
> _______________________________________________
> Forum mailing list
> Forum at mail.gap-system.org
> http://mail.gap-system.org/mailman/listinfo/forum