>
> http://axiom-wiki.newsynthesis.org/SandBoxBugGroebnerFactorize
>
> A simple example of Groebner factorization works:
<snip>
>
> But the following moderately complex problem fails:
>
> --
> ideq2:LIST(POLY(FRAC(SMP(INT,OVAR([mp,mq,mr,yqp,yrp,yrq]))))) := [ _
<snip>
> groebnerFactorize ideq2
> --
>
> with the message
>
> >> Error detected within library code:
> index out of range
>
> The error apparently originates from 'npcoef.spad' in:
>
> --
> check(tterm:TermC,ulist:Vector(P)) : Union(Detc,"failed") ==
> cfu:P := 1$P;doit:NNI := 0;poselt:NNI := 0;pp:Union(P,"failed")
> termlist : List(VTerm) := tterm.detfacts
> vterm : VTerm := empty()
> #termlist = 1 =>
> vterm := termlist.first
> for elterm in vterm while doit<2 repeat
> (cu1 := elterm.pcoef) ~= 0 => cfu := cu1*cfu
> doit := doit+1
> poselt := position(elterm, vterm)::NNI
> doit = 2 or (pp := tterm.coefu exquo cfu) case "failed" => "failed"
> [vterm.poselt.expt, pp::P, poselt]$Detc
> "failed"
> --
>
> It looks like this line:
>
> (cu1 := elterm.pcoef) ~= 0 => cfu := cu1*cfu
>
> causes premature exit from the loop and really should be:
>
> if (cu1 := elterm.pcoef) ~= 0 then cfu := cu1*cfu
>
> in order that poselt be set properly on exit from the loop. But I am
> not entirely clear on what is happening here or why and the
> documentation in the source code is rather minimal.
>
> In any case after fixing the index problem as above, another problem arises:
>
> >> Error detected within library code:
> Table construction failed in MLIFT
>
> in 'mlift.spad'.
>
> At this point I am rather lost, not being sufficiently familiar with
> exactly what is supposed to be happening here. All I see is that 0's
> seem to end up unexpectedly in the list of polynomials.
npcoef.spad is a heuristic to recover factorization if it is sparse
enough. The same idea is described in L. Bernaridin thesis on
multivariate factorization.
AFAICS the problem you describe are really a factorizer problem.
It is probably triggered by nested types. The first thing to
do is to find out which polynomial is being factorized and
what are involved types.
I will look at the poblem. Unfortunately I am busy with other
work and I have limited connectivity, so my response is slow.
--
Waldek Hebisch
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