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Re: Abs and derivative problems

  • To: mathgroup at
  • Subject: [mg14657] Re: Abs and derivative problems
  • From: Paul Abbott <paul at>
  • Date: Sat, 7 Nov 1998 02:10:05 -0500
  • Organization: University of Western Australia
  • References: <71q9qu$>
  • Sender: owner-wri-mathgroup at

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 ??  

Actually, in the trivial example above, you can use

	ComplexExpand[(a + I b) / (c + I d), TargetFunctions->{Re,Im}]

to get what you want.

> I included an example below (cell format, you can cut and paste).

In the Notebook fragment below, I show a simple trick for computing the
complex conjugate of complex expressions with real variables.  The
basic idea is to compute the conjugate using the replacement

	Complex[a_, b_] :> Complex[a, -b]

actually implemented using the built-in SuperStar function (so that you
can compute conjugates by raising them to the power *):

	SuperStar[x_] := x /. Complex[a_, b_] :> Complex[a, -b]

The point then is that Abs[z]^2 = SuperStar[z] z which enables you to
quickly and efficiently compute the type of expression you want.

    \`\(exp = 
      \(b0\ \[CapitalDelta]z\ 
          \((Es - \[ImaginaryI]\ \[Eta]\ \[Omega])\)\)\/\(\(-m\)\ 
            \[Omega]\^2 + m\ \[Omega] - 
          \[ImaginaryI]\ m\ \[Gamma]\ \[Omega] + 
          b0\ \((Es - \[ImaginaryI]\ \[Eta]\ \[Omega])\)\); \)\)],

    \`\(x_\^*\) := x /. Complex(a_, b_) \[RuleDelayed] Complex(a,


    \(TraditionalForm\`\(\(exp\^*\) exp // ExpandAll\) // Simplify\)], 

    \`\(b0\^2\ \[CapitalDelta]z\^2\ 
        \((Es\^2 + \[Eta]\^2\ \[Omega]\^2)
          \)\)\/\(\((Es\^2 + \[Eta]\^2\ \[Omega]\^2)\)\ b0\^2 + 
        2\ m\ \[Omega]\ 
          \((\(-\[Omega]\)\ Es + Es + \[Gamma]\ \[Eta]\ \[Omega])\)\ b0
        m\^2\ \((\[Gamma]\^2 + \((\[Omega] - 1)\)\^2)\)\
}, Open  ]]
> Also,  replacement rules like  //. z[t_] -> t^2 do not work well on
> expressions like  z'[t] + b z[t]. the result  is  z'[t] + b t^2... I
> could not force it to Evaluate z'[t] or D[z[t], t].

One simple, general, and reasonably elegant way is to use pure
functions, e.g.,

	z'[t] + b z[t] /. z -> Function[{t}, t^2]

Another is to explicitly compute all derivatives:

	rule = z[t] -> t^2;

	z'[t] + b z[t] /. {rule, D[rule,t]}


Paul Abbott                                   Phone: +61-8-9380-2734
Department of Physics                           Fax: +61-8-9380-1014
The University of Western Australia            Nedlands WA  6907       
mailto:paul at  AUSTRALIA              

            God IS a weakly left-handed dice player

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