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Topic: Is it me or is it Wolfram?
Replies: 16   Last Post: May 13, 2013 4:51 PM

 Messages: [ Previous | Next ]
 JT Posts: 1,448 Registered: 4/7/12
Re: Is it me or is it Wolfram?
Posted: May 10, 2013 1:37 PM

On 10 Maj, 19:36, JT <jonas.thornv...@gmail.com> wrote:
> On 10 Maj, 19:13, Ray Vickson <RGVick...@shaw.ca> wrote:
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> > On Thursday, May 9, 2013 11:01:54 AM UTC-7, JT wrote:
> > >http://www.wolframalpha.com/input/?i=0.499999999999999999999999999999...
>
> > > n = -1.
>
> > > 0.49999999999999999999999999999999999999999 = (n/2-1)/n
>
> > >http://www.wolframalpha.com/input/?i=%3D%2810000000000000000000000000...
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> > > 0.49999999999999999999999999999999999999999=(100000000000000000000000000000000000000000/2-1)/
>
> > > 100000000000000000000000000000000000000000
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> > > I do not understand to, can please someone explain why and how wolfram
>
> > > get -1 for the upper calculation, it is obvious using the one below
>
> > > what the solution is?
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> > > And if there was two solutions should not Wolfram give them both? What
>
> > > is going on here, i am total newb to math calculators so tell me what
>
> > > is going on?
>
> > It must have something to do with truncation of floating-point numbers. I do not have access to Mathematica, but here is your example in Maple 14:
>
> >   eq:=z=(n/2-1)/n: lprint(eq);   z = (1/2*n-1)/n  <---equation
> >   Nz:=solve(eq,n): lprint(Nz);   -2/(2*z-1) <--- solution

>
> > Default digits setting = 10 gives:
> > eq1:=0.49999999999999999999999999999999999999999 = (n/2-1)/n :
> > lprint(eq1); .49999999999999999999999999999999999999999 = (1/2*n-1)/n
> > solve(eq1,n);
> >                                          42
> >                           0.1000000000 10
> > (this is 0.10e42)
> > subs(z=0.49999999999999999999999999999999999999999,Nz);
> >                            Float(-infinity)

>
> > Even with the low digits setting, Maple handles the direct equation well, but fails when the parameter is
> > substituted into the solution (because the extra digits are handled in one problem but not in the other)'

>
> > Now let's increase the digits setting:
>
> > Digits:=60;
> > subs(z=0.49999999999999999999999999999999999999999,Nz);
> >                                                                   42
> >  0.100000000000000000000000000000000000000000000000000000000000 10
> > (this is 0.100....00 e42)

>
> > Now both ways handle the extra digits well enough to yield identical answers.
>
> > I presume something like this happens also in Mathematica. However, if by "Wolfram" you mean wolfram alpha, I don't know if it allows you to change digit settings easily.
>
> Well honestly wolfram alpha gives answers both for mathematica and
> wolfram, so the cat is eating its tail here.

And i already solved this 15 years ago, so it is not my turn to do it
again.

Date Subject Author
5/9/13 JT
5/9/13 JT
5/9/13 JT
5/10/13 JT
5/10/13 JT
5/10/13 RGVickson@shaw.ca
5/10/13 JT
5/10/13 JT
5/10/13 JT
5/10/13 LudovicoVan
5/10/13 JT
5/10/13 LudovicoVan
5/10/13 JT
5/10/13 JT
5/12/13 JT
5/13/13 JT
5/13/13 Brian Q. Hutchings