Karim BELABAS on Mon, 10 Mar 2003 22:41:09 +0100 (MET) |
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Re: rnflllgram() regression |
On Fri, 7 Mar 2003, Igor Schein wrote: > On Mon, Mar 03, 2003 at 12:48:04PM +0100, Karim BELABAS wrote: >> On Sun, 2 Mar 2003, Igor Schein wrote: >>> ? nf=nfinit(y^9-10*y^8-y^7+7*y^6+y^5-y^4+5*y^2+9*y+4); >>> ? pol=x^9-10*x^8-10*x^7+6*x^6-4*x^5+7*x^4+7*x^3+8*x^2-1; >>> ? rnflllgram(nf,pol,rnfpseudobasis(nf,pol)); >>> *** not a definite matrix in lllgram >>> >>> It was broken some time between 2.2.4 and 2.2.5 >> >> Precision loss. It occured in 2.2.4 also, but was slightly less acute >> (different internal nf format), hence was hidden / disregarded [ and the >> result was incorrect: not LLL reduced ]. >> >> At \p50 it works. > > Any way to have precision insufficiency detected? I mean, the error > seems pretty arbitrary, even I didn't think to try higher precision, > and I should definitely know by now :) Basically, the algorithm doesn't work [ doesn't really reduce as much as desired ]. But you're right, it's no reason to be numerically unstable on top of that. I have fixed the numerical instability (not the algorithm). It also fixes an old TODO item: 4 rnfpolred is numerically unstable: ? rnfpolred(nfinit(quadpoly(904,y)),quadray(904,1)) *** division by zero in gdiv, gdivgs or ginv Cheers, Karim. -- Karim Belabas Tel: (+33) (0)1 69 15 57 48 Dép. de Mathématiques, Bât. 425 Fax: (+33) (0)1 69 15 60 19 Université Paris-Sud http://www.math.u-psud.fr/~belabas/ F-91405 Orsay (France) http://www.parigp-home.de/ [PARI/GP]