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MathGroup Archive 2004

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Re: Beware of adding 0.0

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50245] Re: Beware of adding 0.0
  • From: "paul" <paul at AOI.com>
  • Date: Mon, 23 Aug 2004 06:34:06 -0400 (EDT)
  • References: <cg4fcn$cf5$1@smc.vnet.net> <cg97i2$a62$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Indeed a nice problem Professor!  Thank you for sharing it.  Would you mind
helping us understand something, why is it that when we give the following
input:
In[69]:=
p = 314159265358979323
q = 314159265358979323.
r = 314159265358979323.00000000000000000000;
s = p + 0.00000000000000000000;
{p, q, r, s}
{N[Tan[p]], N[Tan[q]], N[Tan[r]], N[Tan[s]]}

And get the following output:

Out[69]=
314159265358979323
Out[70]=
\!\(3.14159265358979323`17.497149872694134*^17\)
Out[73]=
\!\({314159265358979323, 3.14159265358979323`17.497149872694134*^17,
3.14159265358979323`37.49714987269414*^17, 3.141592653589793`*^17}\)
Out[74]=
{1.60125, ComplexInfinity, -1.12979, 1.60125}

Then, the output for Tan[p] and Tan[q] give two different answers?  Even
though they are almost exactly the same numbers (only different in
precision), If it can calculate the Tan[314159265358979323] then why is it
not able to give an approximate value for
Tan[3.14159265358979323`17.497149872694134*^17], Also, if it calculates
Tan[314159265358979323] to be 1.60125, then why does it give
Tan[3.14159265358979323`37.49714987269414*^17] with such a severe variation
(-1.12979)?

Thanks for your help,

Regards,


"Richard Fateman" <rfateman at sbcglobal.net> wrote in message
news:cg97i2$a62$1 at smc.vnet.net...

> Paul, since you are copying this from my posting to sci.math.symbolic
> of August 19,2004, (yesterday) perhaps you should give me credit.
> I took the example from my own paper, A Review of Mathematica,
> which appeared in J. Symbolic Computation,(1992) and a version of which
> is online. You might also read Mark Soufroniou's INRIA paper (1999?)
> explaining why this behavior is apparently intentional, even if it is
> hard to explain (shown by the responses you got).
>
> The idea that 0.0 should represent all numbers in [-1,1]* 2^{-precision}
> is generally regarded with skepticism by computational scientists. Some
> of the other consequences of this are discussed by Soufroniou and others.
> RJF
>
>
>
> paul wrote:
> > In Mathematica 5.0, I get a very strange result when I add 0.0 to a
number,
> > why is this?  See below:
> >
> > p = SetPrecision[314159265358979323, 25]
> > q = SetPrecision[314159265358979323., 25]
> > r = SetPrecision[314159265358979323.00000000000000000000, 25]
> > s = SetPrecision[p + 0.0, 25]
> > {p == q, q == r, r == s, r == p}
> > {Tan[s], N[Tan[p]], Tan[q], Tan[r]}
> >
> >
> > \!\(3.14159265358979323`25.*^17\)
> > Out[47]=
> > \!\(3.14159265358979323`25.*^17\)
> > Out[48]=
> > \!\(3.14159265358979323`25.*^17\)
> > Out[49]=
> > \!\(3.14159265358979328`25.*^17\)   *****NOTICE the 28 instead of 23 at
the
> > end, why does this happen? *******
> > Out[50]=
> > {True, True, False, True}
> > Out[51]=
> > {1.599808, -1.12979, -1.1297927, -1.1297927}
> >
>



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