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

[Daniel Blazewicz]

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