Prev: Lining up y axes and sizing plot areas
Next: axes with arrows with ParametricPlot3D
From: slawek on 21 Apr 2010 04:30
In[1] := Im[a] ^= 0; Conjugate[a b]

Out[1] := Conjugate[a b]

It is not funny, Mathematica 6 properly evaluates the result

a Conjugate[b]

What happens?

slawek


From: David Bailey on 21 Apr 2010 07:47
slawek wrote:
> In[1] := Im[a] ^= 0; Conjugate[a b]
>
> Out[1] := Conjugate[a b]
>
> It is not funny, Mathematica 6 properly evaluates the result
>
> a Conjugate[b]
>
> What happens?
>
> slawek
>
>
There are several points here:

1) The answer from 7.0.1 is not wrong - just not in the form you desired.

2) Your code assumed that Conjugate called Im internally - this is the
sort of assumption that may vary from one version to the next.

3) Using the code:

FullSimplify[Conjugate[a b], Im[a] == 0]

solves the problem, but in a possibly unreliable way because (a
Conjugate[b]) is actually no simpler than Conjugate[a b]

Ideally, you need to find a way to tell Mathematica that a is Real, and
what kind of expression you prefer, for example:

In[8]:= complexity[expr_] := LeafCount[expr] +
10*Count[expr, Conjugate[x_ y_], {0, Infinity}];

FullSimplify[Conjugate[a b], Im[a] == 0,
ComplexityFunction -> complexity]

Out[9]= a Conjugate[b]

Note that although that looks like a lot of work, you can wrap the
process up in a function that you define at startup, or in a package,
and then use as required.

David Bailey
http://www.dbaileyconsultancy.co.uk

 | 
Pages: 1
Prev: Lining up y axes and sizing plot areas
Next: axes with arrows with ParametricPlot3D