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MathGroup Archive 2007

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Changed order of multiplication in Times[a, b, c] in version 6?!

  • To: mathgroup at smc.vnet.net
  • Subject: [mg77568] Changed order of multiplication in Times[a, b, c] in version 6?!
  • From: Andrew Moylan <andrew.j.moylan at gmail.com>
  • Date: Wed, 13 Jun 2007 07:27:20 -0400 (EDT)

I think the order in which the pairwise multiplication in Times[a, b,
c] has changed between version 5.2 and version 6! Can anyone comment
on this?

Example:

Here are three exact numbers whose product is Sqrt[-1]:

Times[(1 + I)*10^2000, (1 + I)*1/2*10^-2000, 1]

Here are those numbers in approximate form, labelled a, b, and c:

{a, b, c} =
{
1.`15.954589770191005*^2000*(1 + I),
5.`15.954589770191005*^-2001*(1 + I),
1. + 0.*I
};

Here are their various pairwise products:

  Inputting:
{
a*b,
b*c,
a*c
}
  yields the output:
{
0``15.653559774527023 + 1.`15.653559774527023*I,
0. + 0.*I,
1.`15.954589770191005*^2000 + 1.`15.954589770191005*^2000*I
}

So far, all these results are identical between versions 5.2 and 6.

Now consider a*b*c. Here is what is given version 5.2:

0``15.653559774526984 + 1.`15.653559774526984*I

This happens to be the right answer, because it has done the
multiplication in the order (a*b)*c.

Here is what is given for a*b*c in version 6:

0. + 0.*I

This happens to be the wrong answer, because it has done the
multiplication in the order a*(b*c).



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