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Bill Allombert on Fri, 16 Feb 2024 17:09:30 +0100
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Re: Numerical instability in extracting base B digits of the fractional part of real
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- To: pari-dev@pari.math.u-bordeaux.fr
- Subject: Re: Numerical instability in extracting base B digits of the fractional part of real
- From: Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>
- Date: Fri, 16 Feb 2024 17:09:01 +0100
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On Fri, Feb 16, 2024 at 05:55:54PM +0200, Georgi Guninski wrote:
> On Fri, Feb 16, 2024 at 1:20 PM Bill Allombert
> <Bill.Allombert@math.u-bordeaux.fr> wrote:
> >
> > ? a=.33
> > %6 = 0.33000000000000000000000000000000000000
> > ? a*10-3
> > %7 = 0.30000000000000000000000000000000000000
> > ? b=a*10-3
> > %8 = 0.30000000000000000000000000000000000000
> > ? b*10-3
> > %9 = -2.350988701644575016E-38
> >
>
> Thanks. Why I don't see -2.350988701644575016E-38 in my session?
> It must be there, since you unrolled the first two iterations of
> the loop.
Because you continue with A=frac(A), and since A is negative, it is A+1,
so
? frac(b*10-3)
%5 = 0.99999999999999999999999999999999999998
Cheers,
Bill.