Re: Abs and derivative problems
- To: mathgroup at smc.vnet.net
- Subject: [mg14650] Re: [mg14639] Abs and derivative problems
- From: Daniel Lichtblau <danl>
- Date: Sat, 7 Nov 1998 02:10:00 -0500
- References: <199811041847.NAA19370@smc.vnet.net.>
- Sender: owner-wri-mathgroup at wolfram.com
sylvan wrote:
>
> I could not calculate the modulus of a complex expression containing
> imaginary parts in both denominator and numerator with Mathematica. An
> Example:
>
> (a + I b) / (c + I d)
>
> a,b,c,d (real) symbolic variables.
>
> In pratice, this should be absolutely trivial. ComplexExpand is not
> effective.
> How do you "tell" mathematica that your variables are real ??
> ...
In[19]:= ComplexExpand[Abs[(a + I b) / (c + I d)],
TargetFunctions->{Re,Im}] // InputForm
Out[19]//InputForm= Sqrt[a^2 + b^2]/Sqrt[c^2 + d^2]
The TargetFunction option is needed to tell ComplexExpand not to use Abs
(or any of Arg, Sign, or Conjugate, for that matter). Otherwise it will
simply return a result that is correct but in terms of Abs, which,
presumably, you do not want.
Daniel Lichtblau
Wolfram Research
- References:
- Abs and derivative problems
- From: "sylvan" <scd@gopher.chem.wayne.edu>
- Abs and derivative problems