Igor Schein on Mon, 30 Nov 1998 13:05:20 -0500 |
[Date Prev] [Date Next] [Thread Prev] [Thread Next] [Date Index] [Thread Index]
Re: Discriminant of non-monic polynomials |
On Mon, Nov 30, 1998 at 08:05:43AM -0500, Karim.Belabas@math.u-psud.fr wrote: Karim, after applying your patch, > > ? g(pol)=subst(pol,x,2*x+1) \\ If pol is f(x) then g(pol) is f(2*x+1) > > ? poldisc(g(polcyclo(57))) > > *** Warning: normalizing a polynomial with 0 leading term. > > *** bus error: bug in GP (please report). FIXED > > > > ? nfdisc(g(x^12-x-1)) > > *** impossible assignment I-->S FIXED > > ? nfdisc(g(polcyclo(23))) > > *** the PARI stack overflows !!! > > > > *** Warning: doubling stack size; new stack = 8000000. > > ? nfdisc(g(polcyclo(23))) > > *** the PARI stack overflows !!! > > > > *** Warning: doubling stack size; new stack = 16000000. Now it doubles only once. Kevin, I'm just curious, what parisize did you use in 2.0.11 when you tried this one? > > > > > ??nfdisc tells me "preferably monic". How do I interprete this? Does > > it mean that nfdisc will give reliable output only for monic > > polynomials? > > No: we use the obvious (and highly inefficient) change of variable to > reduce > to monic form (it's theoretically possible to work directly with > non-monic > polynomials, but it will be a _real_ pain to adapt the code...). [then > call > polred to try and lower the huge index which we just introduced]. > > It's usually a bad idea to use non-monic polynomials, since internally > we are > going to transform them and work on a different polynomial. It's more > efficient to do the change of variable yourself and reduce the output > via > polred / polredabs. Hence the warning. Hmm, are you saying that I can come up with a better change of variable than nfinit() family of commands performs? Also, would you consider it useful to do the reduction automatically inside nfdisc(). Thanks for the patch Igor